They’re Always Changing Their Plans
Both of you were making plans about how to spend your summer vacation. Just a week before you’re supposed to go on vacation your partner backs out and does not want to go because it just might be a waste of time for them. What they might do is encourage you to go with your friends on a prolonged vacation.
They not only encourage you to do that but they also seem happy about it. Then while you go on a vacation they have the time of their lives and spend it with another person. If you really want to catch your partner cheating, the best course of action would be to return from vacation at an earlier date and catch them in the act.