How can I find out who's using my trusted phone number?

If you mean your Trusted Phone number, then go to Settings > [name on top] > Sign in & Security > Two Factor Authentication. Under Trusted Phone numbers, remove any you do not recognize and add the ones that you want.

If you see a number that you do not recognize it was added after using your Password, and for a new device logged into your account it would also require a Verification Code sent to your Trusted Devices or the Trusted Phone number that had been there previously. If you removed a phone number that you did not recognize, then follow up by changing your Apple Account Password. There is no way to verify whose number is there or who added it. That would be something that can only be done with both factors for authentication.