My top pick for tips on how to catch cheating is this book by Sarah Paul

Know the Signs of a Cheating Boyfriend

How would you know that your boyfriend is cheating on you? Although you may not have hard evidence, you do have a feeling that things have changed. There are signs of a cheating boyfriend that you can look for without having to go to the extreme of hiring a private detective to collect evidence for you. The main thing is to pay attention to details - the little things that you would often overlook - and most off trust your instincts that something is not quite right with the relationship.

The first thing you may notice and this is probably what caused you to become suspicious, is a decided lack of intimacy. Your boyfriend may shy away from touching or make excuses of why you cannot go out together. You may notice that he doesn’t have the same enthusiasm for activities that you both once enjoyed. If you do not live together, you may find it harder to reach him on the phone and he may have unusual explanations of why he couldn’t answer your calls.

Check to see if your boyfriend has a history of cheating on his girlfriends. You may have been one of those women with whom he cheated while he was dating someone else. If so, you can probably tell the exact times when things started to change between you. If you do know some of his ex-girlfriends, you can casually start a conversation about how their relationship ended without arousing their suspicions that your relationship may be in jeopardy.

Unwarranted or constant criticism of things you say or do or the clothes you wear is another hint that the relationship is changing. This is because he could be comparing you to his new girlfriend and is waiting for you to get angry so he can have an excuse to end it all. You should not react with anger, but rather take this as an opportunity to sit and talk about what you perceive to be problems. He may not even realize that he is criticizing you or you may just be taking exception to what he says because you do have suspicions of cheating.

Another extreme of behavior could be that your boyfriend is acting more loving than usual and bringing you gifts, which he has never done in the past. While you do like to receive gifts, if you do have suspicions that he is cheating, you should start to question why all the gifts all of a sudden without insulting him.

You do have to give your boyfriend lost of rope to hang himself with so that you can catch him in a lie or in the act of being with another woman. This means you should take things slowly at first and not jump to conclusions too quickly. He will be careful at first, but if he thinks you are not noticing any differences, then he will get lax and start to make mistakes. The you can move in quickly and confront him with the evidence.