If he doenst want you to know, the minute you will ask him, he will go on special secretive mode and it will be harder to find out. If I had waited ONE day before asking my H back in 2007, I would have had hard evidence in my hands (and probably would have been divorced by now)...
Now that you are suspicious, keep your eyes open and wait. And, TRUST YOUR INSTICT. If it feels, funny, you probably are on to something... Knowledge is power. K