In terms of there being an OW, I'd ask yourself 3 things, first, if something did happen, would that be a deal breaker?
Second, in the state your H is in, do you think he could actually develop a close and meaningful relationship with anyone else? If the answer to that is no, I'd go back to question 1 and ask myself if a PA were a dealbreaker or not.
Third, and probably most important, are you basing your suspicions on anything really concrete, or are you just making assumptions? Looks like from your post that you don't REALLY think there would be anything going on. If you don't REALLY have any reason to believe something inappropriate is going on, it is not good for you to dwell on it. What does your gut tell you? Again if your gut tells you something IS going on, and this is an absolute dealbreaker for you, THEN you could look into it more.
From your tone though it sounds as though an OW would be something to overcome, and not a dealbreaker, so I would do my best to ignore anything that even slightly suggests one, and act "as-if" there is nobody but you.
I have to say in my own sitch, there is one of my H's classmates who I don't trust, and I never have. However he has always told me there would be nobody else, and I choose to believe him. Also this would not be a dealbreaker for me given the state that he is in, i.e. I consider him at the moment to be like a teenager who hasn't quite learned appropriate behavior, so as such I see no reason to ever dwell on possibilities. Well, I am trying to dwell on happy possibilities at the moment, as that's all that can keep me going in this often seemingly thankless process!!!
ITH
Me:34 H:36 M:5 years T: 8 years Bomb: 07/17/08 I want to be separated for 6 months--I don't know what I want the outcome to be S 07/28/08-11/08/08 Living together ~7 months D Possibly busted?!?!!!