"Is It Wrong For Me To Ask To See My Partner's Phone? I need to confirm some suspicions I have."
You can ask them, but you're just showing them that you don't trust them or that you're paranoid and insecure. And looking through their phone won't help you. If you find evidence of them cheating, are you going to break up with them? If you're smart, you would.
But let's say you don't find any evidence of infidelity. Is that really gonna stop you from wanting to go through it again sometime later? Highly likely. It's only a matter of time before you get that urge.
Talk to them about your insecurities or whatever it is that they're doing that is causing you to want to go through their phone. If you can't work on that, if that doesn't improve, then move on. A relationship without trust will never survive.