I found a snippet of an article that describes exactly how you can tell if your man is cheating:

If you ask him and he is extremely defensive and tries to make you feel like you're crazy for suspecting anything, then that's a key indicator. The cheating man does more than lie, he will then try to turn it around and make the woman feel like she's out of line.

Perfect...if your man fits this, he IS cheating. I wish I'd known that.