Infidelity is a serious thing that can destroy a relationship. However, the suspicion of infidelity can sometimes do just as much damage as the actual cheating. Handling the situation properly is key to preserving the relationship.
Before anything else, you must decide how you want to handle it should your suspicions turn out to be correct. If your boyfriend is really cheating and he wants to stay together, will you feel the same way? Can you trust him again? These are things to think about before exploring your suspicions.
While changes in behavior are not signs of infidelity in and of themselves, keeping track of your boyfriend's actions can help later if inconsistencies or repeated suspicious activities arise. Keep a journal of his appointments, nights out and phone conversations.
Suspicion can help a cheating partner, since it tends to manifest through arguments and accusations. This will signal to your boyfriend that you suspect him, and he will try harder to cover his tracks.
Show up unexpectedly to his work, appointments or nights out. You may catch him in a lie or, at the very least, discourage future infidelity.
Do not confront your boyfriend until you have a significant amount of evidence showing his infidelity. If your suspicious seems to be true, do not reveal all your evidence at once. Allow him to attempt to cover his tracks and then reveal more evidence that destroys his lie. This will make it more likely that he will admit the truth.