Most of your concerns can be answered with the "at least one form of intel that he is not aware of" caveat. A hidden keylogger on his computer would turn up a secret e-mail account (and password); a voice-activated recorder under the front seat of his car might reveal a "heads-up" phone call from him to her, etc. Please don't do this without consulting with an attorney first, or knowing what the laws are in your jurisdiction (you can only put a keylogger on a computer that YOU either own, or jointly own, in most states, for example. Ditto for a car).
Of course, if you can afford it, a private investigator is always the best way to go, but they can get expensive.
This does NOT need to be done forever -- just a period of time (two years perhaps?) for him to go thru withdrawal from OW, and to demonstrate his trustworthiness to you.
