Well, we have a good mutual female friend, who keeps in touch with us both, and is supportive of our M. I have held off asking her anything since I started DBing as it was 'against the rules.' To her, I have been presenting myself as moving forwards and enjoying what new life has to offer. I could ask her and ask that she doesn't tell H.
Otherwise, it's asking him direct (not very appealing, but is it best to be direct?) or asking a mutual male friend, but TBH that friend is more 'his' - tho I know he was against the A relationship. The final person is his XW, who again has been very pro our R and against the A. But that just feels a bit weird. Plus, I'm not sure who, if anyone, yet knows of his redundancy and I don't want to go around breaking that news to others...
If you ask a cheater directly if they're cheating, and they're not anymore, they'll say "No, I'm not."
If you ask a cheater directly if they're cheating, and they still are, they'll say "No, I'm not."
Good luck with that, lol.
No, I wouldn't involve anyone else. All you can do is have independent intel in place, and if you don't you can TELL him (as in "Well, I would only be interested if OW were not still in the picture -- and I mean NO CONTACT -- my position hasn't changed on that"), but you can't ASK him.