I'm going to check my boyfriends phone
Why do you want to check his phone?
I need to check if he has been cheating on me
That's private stuff and you shouldn't be looking through it. You need to trust him more and not do things like that to break his trust in you.
I do trust him but I need to make sure since I'm his girlfriend.
If you trust him then you need to let it go. Looking through somebody's phone is an intrusion that most people won't like. It could lead to him breaking up with you.