Is it OK to check your partners phone?

Hi, Is it okay to check your partner/so/gf or bf/husband or wife's phone? In one of my first relations I had a lot of trust issues still and would check my girls phone when in doubt just to confirm or deny my thoughts. Often so just proving myself I'm stupid for not trusting. But I would always be honest about it and it would make her both really mad and really upset I checked her phone. Both when I found things and didn't found things. One argument would be that if the person gets really mad it would mean they have something to hide further confirming whatever thought is in your mind. But is it ok to check once in a while? Especially in a marriage, or should trust always be blindly when it comes to relationships? And if you have nothing to hide why would it be such an issue? Does it offend you when your so checks your phone? TL;DR: What do you think of your partner checking your phone and what do you think of checking your partners phone?
Is it OK to check your partners phone?
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