in my opinion if they have nothing to hide it shouldn't be a problem. If that person is truly faithful and is being honest with you than they won't have a issue with you checking.
Sometimes you need reassurance. If it puts you at ease, do it. Plus gut instinct is always right, if you think you need to you’re probably on to something
I personally am a firm believer that if you need to check your significant other’s phone (whether you’re dating, engaged, or married), there’s bigger problems at hand. My husband and I both have access to each other’s phone at all times. Never ever have either of us snooped or asked to go thru the other’s. My parents on the other hand it’s like a weekly occurrence (been married almost 27 years) but there’s always been problems between them ever since my older sister was a baby so…