My partner claims they are "just friends," but I think they’re having an affair. What should I do?
Do you have good reason to suspect that the relationship has gone beyond friendship? Do you know if any boundaries have been crossed (expressions of affection, secret meetings, physical/sexual contact)? Are they violating boundary agreements you made when you started your relationship/marriage? If so, then common sense tells you that it's not just friendship, no matter what they claim. If they claim they are committed to your relationship, then you should insist that the "friendship" ends and that you both get counseling help to address what has happened. If they resist, or if they say that you have no right to interfere with their choice of friends, then you're going to have to accept the fact that they are giving priority to the other relationship.
What if you don't have evidence of an affair, but something still seems wrong about the "friendship"? Again, be honest with your parther. Don't threaten or accuse; just tell them what your observing and why it feels unsafe to you. Their reaction will tell you much about the health of your relationship and their commitment to it. I've never known a person committed to his/her marriage who wasn't willing to sacrifice any friendship that was getting in the way.
All this assumes that your suspicions are reasonable. If you are a person who is constantly insecure with a history of accusing your romantic partners of cheating, then maybe your fear is a symptom of unresolved personal issues and not your spouse's unfaithfulness. If so, recognize the possibility and seek counsel to gain a balanced perspective.