Spent the first night away from each other after dating for close to a year.
Walked in to his dad's house where we were living at the moment and made a comment about not hearing from boyfriend all day. His dad said "Oh yeah he's asleep in his room, he came back home around 4am, he said he'd taken his Playstation to his friend's and they were gaming all night"
I said "oh that's interesting, because his Playstation is at my mom's house from the last time we were there"

And that was enough for me to suspect something and check his phone. On snapchat he'd saved a chat to a girl he had already been caught flirting with, where he asked her to "please keep this a secret in case my girlfriend somehow finds out and messages you" and then went on to compliment fucking her lmao. Idiot.
Anyway thanks dad!