How to identify my Apple Account

This explains.

If you forgot your Apple Account primary email address or phone number - Apple Support

If this is an iPhone or iPad, look at the top of Settings.

If a Mac, open System Settings and look there.

What to do: If you forgot your Apple Account primary email address or phone number - Apple Support

If this is about a newly-acquired device and Activation Lock, contact the seller and request resolution or refund. It’s a brick until they relinquish ownership.