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

The Infidelity Warning Signs a Spouse Should Be Aware Of

When a spouse has a gut feeling that his/her partner is being unfaithful, there is usually something to back up this intuition. Once you do get this feeling, start looking for infidelity warning signs that will help you deny or confirm your suspicions. Delve further into the action that first aroused your suspicion and you will see there are other signs that you probably missed because this is something you didn’t expect to happen to your relationship.

Inattentiveness to a partner is one of the first signs of infidelity. When you start to feel as if your spouse is not paying as much attention to you now as he/she once did, start thinking about how long this has been going on. This is usually not something that starts suddenly but rather then attention gradually decreases. The length of time it has been occurring will give you a hint as to how long the affair has been going on. Try cuddling up with your spouse or holding hands just to see what the reaction will be. If it is one of disbelief or if the partner does show you some affection and then breaks it off, you should take this as a sign that there is someone else. You can have a conversation about what is happening and gauge the reaction you receive.

Absence from family functions, not wanting to accept the invitations of friends or not wanting to have friends over are also warning signs of unfaithfulness. This shows you that your partner has moved on to other friends and activities to which you are not privy. A sudden excuse, that really doesn’t make sense, for not being able to be home for a certain occasion may mean that your partner is spending this time with a lover.

When you engage your spouse in a casual conversation about extramarital affairs, watch the reaction. An unfaithful spouse will often become very defensive and want to know why you are bringing up this topic. Your spouse may also look very uncomfortable talking about this topic and try to change the subject right away.

When your spouse invents excuses to be away from the home, you should take this as a warning sign. This can include an extraordinary amount of travel associated with a job where previously there was only a small amount of travel involved. Hours and hours of overtime that don’t translate into extra pay is also a warning sign that you should be aware of.

While it is devastating to discover that you spouse has been cheating on you, it is best to find out about it earlier rather than later. When you discover it early, you have a chance of saving your relationship, whereas if it has been going on for a long time, your spouse may decide to move out and sever your relationship all together. What you do need to so is to have open and frank discussions about your feelings for one another and if necessary, go to a marriage counselor to help you make the right decisions.