☛ If she's suddenly spent more time at the gym and is much more interested in getting in shape, she may be doing it for another guy.
☛ Just to be clear -- she may also just start caring about her appearance because she wants to look nicer and be more in shape. But this point, coupled with others, can be a sign of cheating.
☛ If she's cheating, then seeing your family will be a reminder of her disloyalty, and she'll naturally want to avoid them.
☛ If she starts withdrawing from your family, it may also just be a sign that she's ready to end the relationship, whether she's cheating or not.
3. See if she withdraws from your friends. The same goes for your friends -- though she might have previously loved group trips to the beach or to trivia night with your friends, if she suddenly never wants to hang out with them, then it may be because she's trying to extricate herself from you. If she's cheating on you, then she may feel like she doesn't deserve to be in your social circle, or she doesn't want a reminder of how much fun you used to have before things went sour.
☛ If this becomes really obvious, don't be afraid to ask her why. She could have another explanation for why she doesn't want to spend time with your friends.
☛ And when she does hang out with you and your friends and their behavior seems to change and they act all uncomfortable around her, then they may know something that you don't. This is a classic sign that your girlfriend is cheating on you and you are the last to know.
4. See if she withdraws from sex. While cheating men are more likely to have sex even more with their cheating partner, it's less likely for a cheating woman to react the same way. If she's getting some love somewhere else, then she may feel guilty, or just plain not interested, in sleeping with you. If you previously had a pretty good sex routine going, and now she's been "too tired" or "not feeling well" the last ten times you tried to make something happen, it could be a sign that she's getting satisfied elsewhere.
☛ If she's cheating on you, she'll also be less open with her body. She won't want you to see her undress, and she'll be more likely to cover up in bed.
☛ Of course, her not wanting to have sex could just be a sign of her general dissatisfaction with the relationship, or just her unhappiness in her own life. Still, if she's flirty and fun when when she leaves the house, and completely withdrawn with you, it's a problem.
5. See if she stops paying attention to you at all. What's worse than a girlfriend who constantly nags you? One who doesn't care about you at all. Once her interest completely fades, you'll be wishing she was still complaining that you never do the dishes. If she has stopped paying attention to you, asking about how work or school is going, or even noting important events in your life, then she has checked out, either because she's seeing someone else or she's just not into you any more.
☛ Once this happens, it's time to get some answers. There's no point in continuing on if she truly seems to have stopped caring.
6 See if she's suddenly too busy to hang out with you. Were Sundays your brunch and cuddling days, and now she's nowhere to be found on the weekends? Did you watch your favorite TV show together every Tuesday, and now see that she's MIA on those nights? Does she not even apologize or notice that you're no longer doing your usual things together? If she's suddenly slammed at work, and spending a lot more time with friends, and spending a lot more time at the gym, then this may be a sign that another man is really eating up her time.
☛ Notice how she acts when she steps out of the house to "go meet friends." Does she look extra sexy, hum to herself, or just act unnaturally giddy? If so, it may be because she's meeting a special someone.
7. See if she gets more secretive around her phone and computer. Previously, she showed you her computer to point out a funny article or asked you to check her texts for her and always left her phone out in the open. Now, she shuts her laptop any time you come into the room, never sends text messages in front of you, and even added a password to her phone -- or even her computer. If she wasn't previously private and has changed her tune, then she must not want you to see who she's been chatting with.
☛ If you're really feeling bold and catch her texting, you can casually ask her who she's talking to. See how she reacts. Is she jumpy and defensive, or does she give a quick and honest-sounding answer?
8. See if she goes MIA for hours without explanation. This is one of the biggest red flags that your woman is being unfaithful. If you call her or text her and hear nothing back for several hours, then you may have a problem. If it happens once or twice, fine. But if it's happened multiple times a week and she always said her phone died or that she didn't hear the ring, your guard should be up.
☛ When she's MIA for a few hours, casually ask her what she's been up to. Try to read her face: does she deflect you and look embarrassed?
Part 2 of 3: Noticing What She Says
1. See if she's mentioned a "new friend" suddenly. Has a new coworker or classmate's name come up once or twice recently? Has it come up -- a lot? If so, then this may be the mystery man you're afraid of. Try asking some basic questions about this person. If she blushes or gets defensive, it may be because she doesn't want you to know anything about the man she's seeing.
☛ Try to notice her tone or the look on her face when this person comes up. Does she look excited and giddy -- or just guilty? If so, she may be cheating.
2. See if she's nagging you more than usual. Though she may have a lot of reasons for nagging you, one of them may be that she no longer finds you so alluring and that the relationship is on the rocks. She could also be feeling guilty for betraying you, and can be using her nagging to justify her behavior. If she points out every little thing you've done wrong, then she may be doing it to feel better about acting badly.
☛ Again, this could just be a sign that she's unhappy in the relationship. Still, it's not a good sign.
☛ Of course, if your behavior has changed, then maybe she has a reason for nagging you more.
3. See if she's shady about her schedule. If she used to tell you things like, "I'm grabbing coffee with Katie -- I'll be home in two hours," and now rushes off without barely acknowledging where she's going, then something may be up. If you no longer know what she's up to for hours at a time and she's shady when you ask about it, then she may be spending that time with someone else.
☛ If she tells you she did one thing and then later says she was doing something else entirely, then you've caught her in a lie.
4. See if she's suddenly more interested in your schedule. You may think that this is a sign that she really cares about you, but it could be her way of making sure you don't catch her cheating. If she asks you to be clear about when you're getting home and even texts you asking, she may not be eagerly waiting for your arrival. She could also be asking to make sure that you won't be hanging out anywhere near the place where she'll be having her romantic rendezvous.
☛ If you want to try to catch her, tell her you won't be going to a popular spot in town -- like the go-to happy hour bar, for example, and change your mind at the last minute and show up there.
Part 3 of 3: Finding Out if She's Cheating
1. Follow her. Though this is a major breach of trust, if you've reached the point of desperation but are too scared to ask, then try to follow her to see if she's doing what she says she's doing. You don't have to drive off after her car like a creep, but you could try to drop by the locations where she says she's going to be to see if her word is good. Here are some things you can do:
☛ If she says she's having a late night at work, show up with take-out to see how she's feeling. If she's not there, she's in trouble.
☛ If she says she's going out to a bar or restaurant with her girlfriends and it's not too sketchy for you to be nearby, try walking by an hour or so into the meeting to see if she's really there. Think of a good excuse to be in the area.
2. Show up to her place unexpectedly. If she says she'll be ready for you to come over in thirty minutes, show up right away. Say you just couldn't wait to see her. Is she dying to jump in the shower, or to cover up the evidence of another man? Does she not let you come into her bedroom -- or worse, does she not let you come over at all? This is a huge red flag and a perfect way to catch her in a lie if that's really how you want to go about it.
☛ When you get there, play dumb. Just say you wanted to surprise her, or that you misread her text or something, and don't act like anything is up.
3. Look through her things. Again, this is another stellar way to lose her trust forever. But if you're getting desperate, check out her computer or phone if you can, to see if you can access her email or past texts. If she's deleted all of her past texts and emails, then that's a little bit suspicious too. Really, this is a pretty terrible way to go about it, but it is one way to know if she's really cheating or not.
4. Just ask. Tell her you have something on your mind, and ask if she has time to talk in a private space. Don't accuse her (such as, "I know you're cheating!"), but do express your feelings (like "I feel like you're losing interest in our relationship").
Notice whether or not she gives you an answer. If she isn't cheating, she has no reason not to be straightforward and reassure you that everything is fine. If she is, she might talk in circles without really saying anything.
If she becomes defensive or tries to turn the tables on you, this might be her guilt talking.
5. Follow your gut. Sometimes, you just know things without having to second-guess yourself. No amount of crying and denial can rival the feelings you have in the pit of your stomach. After you've taken all the evidence into consideration, it's time to follow your intuition.
Source: http://www.wikihow.com/Know-if-Your-Girlfriend-Is-Cheating-on-You