How can I see if someone else is using my apple id on their device

Check ⤵️

See who has access to your device or accounts

See who has access to your device or accounts – Apple Support (UK)

Manage and use your Apple ID

Manage and use your Apple ID – Apple Support (UK)

Security and your Apple ID

Security and your Apple ID - Apple Support

If you think your Apple ID has been compromised

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

Check your Apple ID device list to see where you're signed in

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

• if you see devices that don't belong to you, delete from your account.

You can go to settings>your name and scroll down a bit. There you can see the devices associated with your Apple ID