I'm going to look through my ex's phone to confirm she slept with my best friend
Why do you care what your ex did?
It will make me feel better about our break-up.
I'm not sure that information will bring you closure. It's never a good idea to invade someone's privacy by snooping.
I'm a man and I can do whatever I want to.
I just don't think it's okay to invade anyone's privacy like that, even if she is your ex. For now on you should just leave her alone and forget about her.