How do I know if somebody is using my Apple ID and iCloud account?

Click on this link "If you think your Apple ID has been compromised" --> https://support.apple.com/HT204145

'Checklist: If you want to see if anyone else has access to your device or accounts' click here --> https://support.apple.com/HT212021

Security and your Apple ID - https://support.apple.com/HT201303

Contact Apple for help with Apple ID account security - https://support.apple.com/HT204169 "This article provides country-specific Apple Support contact information for customers seeking help with their Apple ID password or other security-related issues." Better for directly reporting an issue.

The best way is to use the security measures Apple already provides:

Enable 2FA, maintain positive control of your devices …

.. and NEVER EVER share your AppleID password and device Pass Codes w/ ANYONE.

If you’re at all in doubt, change them now and read this:

https://support.apple.com/en-au/HT204145

Finally learn more here:

https://support.apple.com/en-us/HT201303