How do I know he cheated on me?

Victim of being cheated on twice, this was my experience;

He started being to tired to come near me (sexually) obviously worn out by the girl his been with. He become less attentive and careless and thought I was crazy for thinking he was cheating, but then other times he was to too nice and I grew suspicious, but he could be angry for other reasons, but you know your guy, do some digging and yes following wherever his going it may be, no your not a stalker it's your boyfriend, and your having doubts. Ask people, ask girls.

His young, likely he could be, but don't worry if he has, his loss, your obviously a good girl and if he has he'll realise what his lost.

But I hope he hasn't, I know it hurts, it's not nice but good luck