How do you know is someone is on your account?

Check your device list to see if there is a device signed in that you don't recognize. You can remove the device using the steps in the article below. As a precaution, change your password.

Apple ID device list -Check to see where you‘re signed in.   * 

 Apple ID Password Change.   -  

ASC YouTube for How to reset your Apple ID password

 What to do after you change your Apple ID or password - Apple Support  -   I suggest doing this before you change your Apple ID or password.

Hello Gxx1313,

To check what devices are signed in to your account go to appleid.apple.com, then select devices if they do not pop up there click more info then click on the device names to see info and remove them from your account

Hope this helps!!!