First time, (age 20 and stupidly obsessed with a much older man) was the standard I was using laptop and found emails from Fab Swingers. He swore blind it was one off, meant nothing, etc, and I blindly stayed.
Suspected for years after that, never really trusted him again. So many red flags in hindsight - phone always locked, put face down, password changed every time he thought I’d guessed it, went in shower with his phone, etc. Disappeared incommunicado on many a night out. Any excuse to walk out in a huff. He worked nights and friends used to tell me they’d seen his car parked outside some dodgy places in our city.
Shortly before we finally broke up I paid for a weekend away in a posh hotel. He was moody and short and snappy the whole time, wouldn’t do anything or go anywhere; ended up in the room watching TV despite a spa and bars and restaurants on site. He went to the loo and forgot his phone and I accessed it and found browser tabs open where he had been searching local escorts etc 
When we broke up officially, I was out a few days later and someone told me in a pub that he’d been shagging their mate for six months. He ended up in a relationship with her for a year or so after that before she finished with him.
They did me a massive favour - within a year I’d met my now DH and discovered how it felt to be treated properly.