Someone is using my iPhone's Apple ID account

If you change the Apple ID password on your device then all the devices that do not belong to you will automatically get logged out and will not have any link to your device.

Thumb Rules (Keep in mind always):

1. Enable  Guided Access before handing over an iPhone to kids
2. Don't share Apple IDs
3. Don't Jailbreak
4. Don't share sensitive information pertaining to your device and payment methods
5. Beware of Phishing
6. Do not share your location with a stranger --> Find friends and share your location with Find My
7. If you see this --> What to do if you get an alert that an AirTag, Find My network accessory or set of AirPods is with you
8. Do not leave your device unlocked or without a passcode unattended in a public place --> If you think your Apple ID has been compromised

Change your Apple ID password

1. Tap Settings > your name > Password & Security.
2. Tap Change Password.
3. Enter your current password or device passcode, then enter a new password and confirm the new password. Forgot your password?
4. Tap Change or Change Password.

Change your Apple ID password - Apple Support (IN)

Sign out on your iPhone, iPad, or iPod touch

1. Go to Settings > [your name].
2. Scroll down to the bottom.
3. Tap Sign Out.
4. Enter your Apple ID password and tap Turn Off. ... 
5. Turn on the data that you want to keep a copy of on your device.
6. Tap Sign Out.
7. Tap Sign Out again to confirm that you want to sign out of iCloud.

Sign out of iCloud on your iPhone, iPad, iPod touch, Apple TV ...

You can only find out if they have logged in presently, not if they have logged out.

Settings > [Tap your Name} > Device will appear here...

See the pic below