Do you think she's cheating on you with a specific person? If so, be weary when she says she's going out and that she cant see you. Go and stand outside his house(out of sight) and see if she shows up.
If not, check through her phone.
I do not however condone this at all. If you don't trust her then leave her. I really do hope she isn't cheating on you though. I weant through it and it hurts your pride as a man, and i struggle to trust girls now.