Back before mobile phones this (the very early 90s)
I was engaged and living with him and had been for a while. All of a sudden he starts going out on a Friday night with his best mate. Didn't really think anything of it, but at the time it was out of character. Then he'd be either late home from work, or bang on time, but would change and go out again 'with his best mate'.
I knew the best mate, and he'd pick him up, so initially, no suspicions although I was getting cheesed off that it was happening so often and he wouldn't let me know in advance so that I could make my own plans, it was always the last min.
One day he went to work without his wallet. I only noticed as I walked past the table and fell on the floor, open. Inside was a condom. No need for him to have one in his wallet. Later that same month I found an earring that wasn't mine in the bed, and he'd turned me down for sex a few times too. The saddest thing, other than the actual cheating, was that he'd recently bought himself new underwear, and new aftershave and had even changed from glasses to contacts for evenings when he went out, and odd days here and there. As I watched him getting dressed one evening (we were chatting, all very friendly) I noticed the new stuff and joked. 'If I didn't know better, I'd think you were having an affair!'. I was so naive, it's something that happens to other people, right? Wrong.
Unfortunately, it's now made me highly suspicious and my poor fella now has to deal with that.