Using your number on other devices and pretending like they’re you, but it’s not how do you find that on your phone or how would you know if they did have one and get rid of it?

No one else can control or access your iPhone, unless you have provided them with your Apple Account and Password among other things.

What is it that you see on your iPhone that makes you think that someone else has control of your iPhone?

Hopefully, you are not asking about a situation where someone has tricked you into installing their Apple Account and Password on your iPhone?

Or, are you just asking a hypothetical question like how do you know that an airplane will not crash on your street. Sure, it's possible. But what are the odds of this ever happening?