If you REALLY want to check your partner’s phone then you should just break up/divorce them.

My unpopular opinion is that checking your partner’s phone is a sign that the relationship is doomed. Either you have unhealed trauma that has caused you to have deep insecurity that compels you to invade your partner’s privacy or your partner is behaving in a way that has demonstrated that they aren’t trustworthy. Either one of those scenarios spells doom for a relationship. Checking their phone doesn’t prove that they’re not cheating on you, because cheaters can just hide the evidence of their cheating. It proves nothing, so that thing that led you to believe that you may find evidence of their cheating won’t go away if you look through their phone and don’t find what you were looking for.

You will still be battling with that insecurity and/or untrustworthy behavior.

If you trust and respect your partner then you will respect their right to privacy. It’s like reading all of their mail, reading their diary, and tapping their phone so you can listen to their private conversations. Not cool behavior!