How do I find out who is using my Apple ID

Your Apple ID should be exclusively yours. No one else, not even a close family member or friend should be using it.

To prevent unauthorized use of your Apple ID, Use two-factor authentication for security on your Mac, iOS devices, and iPadOS devices.

You may want to Change your Apple ID password. Then, read What to do after you change your Apple ID or password.   

I think someone has used my Apple ID on an old machine that I sold. Is there a way to determine when my id was last logged into the machine so I can determine if it was presale or not?