how to determine if my MacBook is in a data breach

Welcome to the Apple Support Communities.

Your Mac cannot be, but an account of yours could be. See: Password security recommendations - Apple Support.

Jack

hnichol Said:

"how do I determine whether my MacBook has been in a data breach"

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You tell us. What makes you suspect this?

Use a New User:

If you are concerned, then create a new administrator account, transfer everything over to it, and then trash this current user.