How do I know if someone is signed into my Apple ID on another’s device?

Hey there,

See ☞ Check your Apple ID device list to see where you're signed in - Apple Support

You might also find this helpful:

Checklist: If you want to see if anyone else has access to your device or accounts - Apple Support

Hi Kellzzzz18,

You will have two notifications in order if you have someone trying to sign in to your account; the first sequence will be iCloud sending a notification to all your devices with an approximate location of the person trying to sign in. You have the option to approve or block the log in. The second sequence will be an automated email sent by Apple informing you of someone trying to sign in. If it wasn't you that signed in, Apple urges that you change your password immediately as it will sign out all devices the moment you change your password.

But since you are asking if you want to know if someone is signed in to your Apple ID, you can check this first and immediate method; through the Find My application. If you find a device that's online and actively moving in a place that's not your house or not familiar to you, your account has been compromised and someone has access to your account.