"Without going into detail, I will know soon enough if you're lying to me, so please tell me the truth -- I think I deserve that much. Are you still in contact with her?"
Chances are still way over 50/50 that he'll lie to you, but a "yes" will tell you a helluva more than a "no" will, and you may just get the "yes." Other than maybe Saddam Hussein, who for some sick reason seemed to WANT the world to think he had WMDs, I can't think of any reason why a cheating spouse would lie and ADMIT to contacting their affair partner. They'll lie the other way around -- DENYING it -- but rarely will they lie in this direction.
Do you really have NO independent way to verify it? If you don't, this is the only way I can think of -- to bluff that you do. Maybe he'll blink.
