My gut was screaming for over 2 years - no proof and I was snooping.
One night when he didn't get home till 2-3 am and I was laying in bed waiting for him I asked him "if our DD was being treated by her H the way you're treating me, what would you advise her?" His answer "leave him the ch-----". I didn't hear the rest, but I thought he said cheater.
It wasn't until I'd started pulling away and concentrated on kids, did he start slipping up and I had circumstantial evidence at best. I told him to leave and he did.
After the divorce was filed I cracked the password on the family computer to find all the evidence I needed that my gut was right!