There are so many variations imo on this.
Firstly if you trust your partner and there are no suspicions then you should not snoop.
I know some people are very open with phones, fb etc with partners and it's all above board with both partners feeling they have nothing to hide and a male friend of mine said he would rather give his dp complete access to everything than have her worrying about anything.
It could be that everything is fine with your partner but you have anxiety, but in every aspect they are still good to be around, attentive, pleased to see you. This could be more to do with you than them doing anything and I don't think it warrants snooping.
Then there is the gut feeling. May be that their behaviour has changed slightly, or you are worried about the way the mention someone, there's been a slight shift in how you connect , at this point you ask them, if things don't improve but you still feel there's something then snooping is possibly a plan.
If they are always on their mobile, have locked it tight, hide the screen, start wearing hair new way, smelly', often put the mobile face down are grumpy at you, brought you home a newspaper clipping about secretary (stupid exh 🙄), then they are sure fire indications that something is going on and snooping is legitimate!
There are always signs and sometimes you don't see them until it's too late or (in my case with exh I was so busy I didn't have time to acknowledge them.
Tapping someone's device is not cool it is illegal and I think you have to ask yourself a) do I already have enough info? And if so then stop hurting yourself and b) remind yourself that this info gained cannot be used with out them knowing you have tapped.
I snooped on two people I had relationships with both had been abusive, one mentally, physically and everything in between. The other mentally abusive, both these me had cheated on me several times each and I just needed something that proved it as I was being told so many things I felt I was going a bit loony. Every single man I have ever had a relationship with (usually long term) has cheated on me, every single one! I have trust issues amongst others.