If you want to find out if your boyfriend cheated on you, you should pay attention to a few key behaviors that may confirm that he was unfaithful. Of course, there is a major difference between behavior that indicates that he may be cheating and concrete evidence that he is cheating. Don't assume you've "proven" him to be a cheater unless you have the latter, as the former can be caused by a number of other reasons.
Sudden Changes May Signal Cheating
Look for sudden changes to your usual routines. While routines do change, they are normally gradual changes. For example, it is not uncommon for a couple who has been together for a long time to have sex less than they did when they first got together, but it is less common for sex to immediately dwindle from, say, three times a week to once a month.
Look for Signs of Other Women
Look for signs of other women. The scent of a different perfume and lipstick on his collar are telltale signs, but you should stay alert for other signs as well.
Ask him routine questions and gauge his response. Someone who is cheating will often become more defensive than someone who is not.
Ask him. This is the most effective way to find out, and it is more effective than any other indicator. You should use the other indicators when you ask him, though, rather than just assuming that he is cheating based on them.
If It's True
Whether he comes clean on his own, or you have undeniable evidence of his infidelity, the best approach is to remain calm and determine the next steps. You need to decide whether the relationship is worth saving. If you do want to salvage your relationship, counseling may help you and your boyfriend discuss and work through your problems. Getting to the root of why he cheated may take many months and a lot of work. If you decide to remain in the relationship, you must also be willing to forgive and move forward.
Don't overly violate his privacy by going through his email, phone or other personal things. If you do go through these and find something, he will be extremely upset that you violated his privacy.