Abbey - can you afford the phone counselling sessions? Those could really help you to get a plan in place to deal with this. Don't expose yourself to potential stds. Don't snoop - what is that going to accomplish? Unless you are trying to get documentation to use in a divorce proceeding, you're only going to make yourself nuts. He has a secret cell phone, he's probably hooked on this woman like she's a drug right now. He knows you probably know, the more certain he is that you know, the more tense things will be as he waits for the other shoe to drop and a big fight to start. You'll have to decide what to do, if you know and he knows you know (which at some point on the path you are on, he will), he may start making moves and you may lose the chance to decide what comes next. Good luck!
Faith is, at one and the same time, absolutely necessary and altogether impossible. --Stanislaw Lem