Don't bank on finding out for definite any time soon. Here goes, I've also told my story before...
Picking arguments and distancing from me and dd. Inc one time in my face telling me he was going to make me end the marriage and make it look like it was my fault.
Mentionitis but about 2 colleagues. One of whom I was v friendly with and initially confided in about my concerns.
Went from being a clock watcher who never worked a minute more than he had to, to going in early, staying late and going in at weekends/back again in eve supposedly. But when I phoned I'd get other colleagues who'd tell me he wasn't there, this was what made me really suspicious.
Luckily I had a wise relative who had not been cheated on but had more life experience than me and had seen it happen to others. She counselled me to play the long game, get ducks in a row, get SHL etc.
Started to be more concerned about his appearance. Stopped complaining about lack of sex, I'd withdrawn marital relations as soon as I had any suspicion for the sake of my health, he'd always been very anti-condoms.
His phone and email suddenly had pass locks when they never had before.
Attempted to keep his phone with him at all times.
Answered home phone as much as possible.
Whenever I even suggested marriage in trouble gaslighted me to point I genuinely thought I was going to end up sectioned. I actually think now he was aiming for this so he could get dd as stuff was moved around, appointments for dd changed without my knowledge etc.
Work functions were suddenly all employee only.
He forgot his phone, I sussed the pass code (wasn't hard), and saw texts between them, but I think I was STILL hoping I was wrong, didn't tie up with certain timings with ow 1(remember the mentionitis?)
Culminated in him helping out ow no 1 With something that involved a trip just a couple hours from us, he called at teatime to say it was 'too far away' so he was staying there for the night. Gave him an ultimatum at this point of come home or we're over. He didn't come home and turned his phone off. When he came home next morning his stuff was on lawn in black bags.
Ow1 couldn't look me in the eye.
Another colleague saw me in the local corner shop and let something odd slip, that's when I realised there were 2!
Ow1, supposedly mine and ow2's friend was basically just a fb and kept her part quiet from both me and ow 2. Ow 2 is now wife 2. She still doesn't know about ow1.
Wife 2 was pregnant at time of split, I learned that sbexh was dithering over ending it with her and was told that of course the person telling me couldn't know for sure but that the timing of the pregnancy was very convenient.
Sbexh literally cried on my shoulder about the pregnancy, and said it was only ever meant to be a bit of fun. At various points he's come sniffing round me, including the day before their wedding. He's on affair no3 since they've been together, pregnancies coincide with him straying. Baby 4 due soon.
It was when he called me drunk trying to get back with me one time I found out the whole truth. So glad I withdrew marital relations, at one stage he was shagging all 3 of us. He was at the time both of theirs' boss and it was a small office.
He's screwed up his career as well as everything else, has no relationship with dd, and I suspect doesn't really want to be with wife 2 or even with anyone.
I'm still single (but not a nun 
) but dd and I have a pretty good relationship (allowing for fact she's now a teen), I have my little flat and a peaceful home life mostly, much better off without him. I certainly wouldn't be in wife 2's position for all the love and money in the world.
OP while your situation sounds similar to mine, I know if at least one similar in my circle and it wasn't an affair it was redundancy being threatened at the guys workplace and he felt in the current economy and at his age he wouldn't get a job and even if he did nowhere near the salary and so was worried about mortgage etc. Not that this excuses bad behaviour but its a possibility.