How do you know he’s cheating? If you find a condom wrapper in your bedroom, and you and your man don’t use condoms, you have your smoking gun. If you find a pair of panties you know are not yours… another smoking gun. But not all cheaters are that careless or stupid! Some of them are very stealthy, and they take great care to cover their tracks. To catch them, you have to pay close attention but not let on that you are suspicious. If they think you’re onto them, they will be even more careful not to leave any evidence.
Let him think you are completely oblivious to what he is doing. That will probably be difficult, because you have to pretend to be happy (or at least, content) and act as though you trust him.
If your man knows you are suspicious, or if you’ve already confronted him, back off! Don’t mention it at all! Then after a while, gradually warm up to him. Here’s the hard part–apologize to him for being suspicious. I know. That’s asking a lot, but remember that getting him to let his guard down is the first step toward finding the truth.
There are probably hundreds, if not thousands, of indications or warning signs that your partner COULD BE cheating. Obviously, I won’t be able to list everything, but I can give you a good sampling of them here. You’ll get the picture. Watch out for the following:
1. Changes in sexual technique–Does he suddenly seem different in bed? Does he have some new tricks? Does he seem more aroused during sex (he might be fantasizing about someone else)? Has he recently started talking about having a three-way with a particular person?
2. Changes in mood– Has he gone from upbeat to grouchy, or from bummed out to happy?
3. Changes in routine–Has he begun to “work late” more than usual, or is he going out “with the guys” a lot lately?
4. Changes in hygiene–Has he suddenly shaved facial hair he’s had for years? Has he grown facial hair he’s never had before? Has he changed his hair style? Is he shaving in places he didn’t shave before? Has he changed cologne or aftershave? Does he wear cologne now, but didn’t before?
5. Attention to fitness–Is he suddenly trying to tone up or lose weight?
6. Attention to style–Has he bought new clothes or shoes, especially of a different type or style than he normally wears? Has he recently bought new and/or different types of undies than he usually wears?
7. Attention to details–Is his normally dirty car or truck now spotless? Does his tie now match his shirt? Is his normally junky apartment or house now presentable?
8. Appearance of being single–Are pictures of you or the two of you no longer visible in his home or office? Has he changed a voice mail message from “we” to “I,” or from “us” to “me”? Does his profile picture on social media no longer have you in it? Or even worse, has he removed pictures of you and/or the two of you from his Facebook page? Does he say flirty things to other women, or put flirty comments in his Facebook status? Has he recently changed his relationship status on social media “single”? Don’t buy into the excuses for that one–you know what it means.
9. Becoming secretive–Has he recently changed passwords on email, voice mail, credit card accounts, etc.? Does he leave the room when receiving a call or text? Does he hang up the phone or close (or glass) computer windows when you walk into the room? Does he hide his phone from you?
10. Becoming “modest”–Has he started to hide his body from your sight, like quickly covering with a towel or clothing if you walk into the room? Does he now want the lights off when you have sex? Has he recently had unexplained scratches on the shoulders or back, or “bug bites” in odd places?
11. Disappearing–Does he say he is going to Walmart, but doesn’t come back for hours, and Walmart is only ten minutes away? Does he say he is playing golf, but doesn’t take his clubs?
12. Pulling away–Does he seem distant? Has he stopped kissing you? Does he resist your sexual advances? Has he stopped saying, “I love you”? Have the “I love you” calls stopped?
13. Hiding you–Has he stopped (or never started) taking you around his family or friends? Has he stopped taking you (or going with you) out in public? Does he say, “I watched Idol last night” to a ‘friend,’ when he watched it with you?
14. Changing habits–Has he started doing his own laundry, when you used to do it? Has the little pile of receipts on the bureau suddenly found a secret home?
15. Condoms missing–This could also go for any sexual aids or devices. Does the KY Liquid seem to have a leak? Has your vibrator or shaver disappeared from his apartment?
16. Inexplicably clean–Does he come home from working all day or hanging out with the guys smelling like he just got out of the shower? That could because he’s been with someone else and showered to remove the evidence.
Okay, you get the idea. As I said, this list is far from being all inclusive. (There’s more info in the Cheaters & Broken Hearts book!)  However, it should give you enough info to get you started on your journey toward the truth
So you’re convinced that he’s cheating. What if he wants to have sex with you? If you really think he is seeing someone else, you should protect yourself from possible exposure to STDs. If you and your man use condoms, you’re probably okay. As you know, they don’t protect 100%, but used properly, they give good protection. 
A ‘headache’ might work a time or two, but then he would know something’s up. You have to find another strategy.
Ladies, if you’re on the pill, tell him you forgot one, so you need to use a condom for a month because it might not be safe. Also, antibiotics reduce the effectiveness of the pill, so you might be able to use that one.
If you’re using an IUD, tell him you’re spotting, and you’re afraid it might be coming out (it does happen), so you need to see your doctor first. If you take the birth control shot, tell him you have a friend who was taking the shot and got pregnant (that happens, too), so you want to use condoms until you can see your doctor, who is booked for the next six weeks.
Or you could say that you think you might be getting a yeast infection. If necessary, use the seven-day over-the-counter cream. It won’t hurt you. After that, say you’re still having symptoms, and you need to see your doctor before you have sex, and of course, the doc is booked. 
Next step.  Get a notebook and find a place to hide it where it could NEVER be found. In this notebook, write down everything you find even remotely suspicious. The more you write, the more real it seems, and the less you’ll doubt yourself when he says you’re crazy, which is the standard denial of a cheater. They usually don’t even bother with, “No, I’m not cheating!”
I think that, “You’re crazy!” is the standard response because he can at least feel as if he isn’t lying–he is deflecting. It sounds pathetic (because it is), but it’s just one of the rationalizations people use–“Well, I never said I wasn’t cheating… at least, I didn’t lie, right?” PLEASE!
If you’re convinced he is cheating, you need to get proof. (See  Getting The Evidence.) 
Be careful, be diligent, and be stealthy. And good luck!
DISCLAIMER:  Any one, or even a few, of these “signs” is not necessarily a guarantee that your partner is cheating. Sometimes people try to make improvements to themselves and their lives, especially as New Year’s resolutions, etc. For this reason, it is important that if you suspect your partner is cheating, find the evidence before confronting him, making any decisions about moving out or making him leave, or going to an attorney.