I guess one of the best ways to tell that he is being honest is the way he is acting.

Does he act short or angry with you still?
Does he have unaccounted time?
Does he leave the room often, and disappear for a few minutes at a time?
Does he contact her at work using a new FB, or email account? His boss may be able to help on this.
Does he talk admirably about the OW? (like she is such a good person she wouldnt do that....blech)

There are signs and red flags that will still announce that he is still in an affiar even if he did do all those things. Still use this time when he is willing to work on the marriage to work on the marriage. Show him you do care about him.