How do you tell if someone is on your apple account

If you believe that your AppleID account has been compromised, follow this guidance to secure your account:

If you think your Apple ID has been compromised - Apple Support

You remember letting someone use your iPhone for an hour or two recently and you also remember giving them your Apple ID and Password.

Strange things have begun to appear on your iPhone.

Another way to tell is to go to Settings > your name at the top.

Scroll down and see what devices are using your account. Any you don't recognize would be an intruder.