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

Emotional Cheating and Physical Cheating - What is the Difference?

When you have a strong emotional attachment to another adult that does not include any sexual intimacy, but you try to keep it secret from your partner, then you are guilty of emotional cheating. You could come back and say you are not really cheating because there is no sex involved, but if your thoughts are consumed with this other person and you exclude your partner, then it is cheating whatever way you want to look at it. Another term that refers to this is having an emotional affair.

The time you spend together may be chaste, yet intimate at the same time. The intimacy may be inappropriate in that you share secrets about you are your partner that you would otherwise never tell anyone. It also involved spending excessive amounts of time with this person that you would ordinarily spend with your partner. Maybe you confide more in this person and you do hold hands and touch each other a lot. You have more feelings and emotions toward this other person that you do for your existing spouse or partner.

When you are involved in an emotional affair, you have the same habits as a person who has a lover. Deception and secrecy take over your life as you keep secrets and devise ways of getting out of the house to meet. You may make up lies about where you are going or where you have been so that your partner doesn’t get suspicious. In your conversations, you are very careful not to make any mention of this other person’s name or if he/she is a friend of both of you, you try not to bring the name into the conversation too often. Even if there is no sexual activity, the fact that you are hiding this relationship or friendship means that you do believe that you are doing something wrong.

When you think about your emotional relationship with this person, you deny that it is an affair. You may even deny that there is any sexual attraction there. Such an affair may go on for a long time, even forever, without any sexual activity being involved. In most cases, though, it does lead to further developments in this area after you spend a lot of time with this “new” confidant. This usually happens between a man and a woman because even though men and women have best friends of their own gender in whom they do confide, such relationships don’t usually blossom into a lesbian or gay relationship.

Cheating on your spouse emotionally is not the same thing as having a special friendship with someone. It can only be called cheating when you don’t tell your partner about this friend, what you do together or where you go. Your partner may never even know that this person exists and it is in not telling or sneaking around that makes it cheating. You will tend to take on the characteristics of a person who is having an affair whether or not there is any sex.