When it comes to the choice of whether you should check her phone or not, why? Usually, there are only two reasons, either she is showing red flags, or you are being naturally paranoid. Both of these factors imply that the relationship is lacking trust, which is something that is essential in a relationship. When you assume shes not cheating by checking her phone its not because you trust her, its because you know she's not by checking everything, and even then, you will never fully know. If she's giving you red flags that spark this feeling in the first place, the relationship is already as good as over, or is very nearly meeting its end. If it's natural paranoia, you are unfairly taking out your insecurities on her. Paranoia is like an addiction, giving in only temporarily gives you a quick fix, and never gets rid of the issue, but increasing your chances of making it worse.

Keep in mind this act can be neccisarly, but before you pursue doing it, wonder why you are contemplating it in the first place, as it might be a symptom of a major underlying issue in the relationship, or in you.