Hard one that.
A friend of mine was cheated on by her now ex husband and she no idea. Even looking back she didn't see how he had time to cheat but it turned out it was someone at work so they'd been seeing each other during the day round work.
I also came across a partner of a friend I used to work with and who had just had their baby all over another girl from where I used to work. Had a horrible time wondering what, if anything to do about it and, although it turned out that they had actually split at this point, some other friend's advice was to send an anonymous letter.
I've also caught someone cheating on me...bloody hell, I'm just pessimistic through experience!
I'd have to have some kind of resolution to it, I just couldn't leave it and forget about it.