Well, it *could* be that the card was meant for someone else, and somehow it wound up in his pile of cards. If it doesn't directly address him, it's kind of hard to have a "smoking gun" evidence-wise.
Not much you can do about the past, catching and frying- so to speak - a cheater needs to be done in the present. I knew my first husband was cheating and he had a porn addiction, so the signs were big and ugly. But something like this, unless you see it in the immediate future, it's really hard to peg. He may very well have cheated long ago, but you gotta ask yourself if the marriage is worth maybe going to counseling for, and a deeper introspective into your relationship as a couple.
So, unfortunately there is no real "yes he did!" or "no he didn't!" answer.
