How do I find out if my iCloud is being used in another device

Log into https://appleid.apple.com/account/manage

Then in the Devices section click on your device.

To remove a device, click on the Device name and then click Remove from account.

Or go to Settings > [username] > [device name] on an iPhone or iPad.

Or go to Apple menu > System Settings > [username] > [device name] on Mac.

Go to settings>your name>security>change your password and select log out of all devices

You can also go to settings>your name> scroll down a bit to see the devices you're currently logged in to.