It's something that (possibly) happened in the past, denied by OH, but there are a few definitely suspect details and he has lied to me in the past about things. It's also not the only thing which is in doubt, but I can't prove anything.
The only contact I have would be over FB. We live nowhere near her now. There would be no repercussions on her at all, but I really would like to know for my own sanity. How could I word a message so that I don't sound completely crazy?