How do I Find out if someone else logged in

On your iPhone or iPad, go to:

Settings -> Apple ID (Your Name)

At the bottom you can see the devices that are signed in to your Apple ID, and therefore also your iCloud.

You can read more here:

https://support.apple.com/en-us/HT205064

If anyone else is logged into your Apple ID, you have a problem. You should immediately change your password.

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