How to check if someone is using my Apple ID with other device?
How to know someone is using your account with other devices
[Re-Titled by Moderator]
We can’t (being fellow users) but you easily can - AFTER changing your password as already recommended - simply by logging-in to https://appleid.apple.com/
Verify everything listed; paying particular attention to “Devices” and “Trusted Numbers”
Delete any devices which you don’t recognize; or numbers which you don’t explicitly trust.
