How do you know if your man is cheating?

It’s been a week already since that argument I had with my boyfriend. I saw him creating an online profile to this foreign dating event, which made me mad. That was our very first misunderstanding and trust me, I’ve never been that furious towards him. That was the reason why I kept silent for days. I trusted him, but he just wasted my trust. It breaks me apart to think that he might be cheating on me with someone else. How do I know if he really is though? I need advice.