I'd tell you how you'll know he's cheating and by that you can figure out if he's cheating or not by considering the very acts of a cheater below...

He'd start off by reducing the length of those sweet love messages he normally slips through your DM: just like old times when your relationship with him was only few weeks old. He'd channel most of those messages to his newly found crush.

He will suddenly begin to get angry real quickly over trivial (little) matters he shouldn't...