Well, it would depend on what it was, how I found out, and how much I knew.
Recently when I visited my boyfriend I was on his computer checking out my bank statement. I ended up seeing something else, it wasn't bad and I wasn't upset, but it did make me wonder. I brought it up a few days later, we talked about it, and everything was fine. (hmm... this makes it seem like I saw bookmarks of farm girls gone wild or something
)
Now, I could still think about it. Stress about it. A younger me would have. But I'm not going to do that to myself or to him. There just isn't a point in it.
If you're really worried about it, and if you think you can't just get past it, talk to him about it. That's the only way to resolve issues in a relationship.