Don't fully trust your partner
You can have all his social media accounts and his phone open all the time but that doesn't guarantee that he won't cheat on you. Mature relationships require a solid foundation of trust and if you find yourself needing to know what your partner is doing all the time, then you don't fully trust your partner.
Be with someone you can feel comfortable not knowing everything he's doing because that means he's a trustworthy person you can fully believe in
Unless he or she GIVES you REASON to be untrusting .... then not trusting your partner is generally a reflection of your own insecurities in the relationship.