There are a number of really useful online resources, both Free and Paid For, where you can research an individual or even a company, to see if an email address, mobile number, a username etc, have been linked to any hacked details.
So, you could search via a Name or Address to locate any linked Email addresses, or search via a Password, to locate accounts using the same Password, both of which may open up your investigation or research.
As well as linked email addresses or mobiles, the results may provide other useful leads and additional information into a subject and indicate which online sites they were linked to when hacked, and so where they have an account etc.
Hacked Email & Online Accounts Checking Sites
Customer Service Details (if your email account has been taken over)