With what we call a "whois", you get the registered information about the owner of a domain. But since a few years now, most of this information is private, one way or another, which is great for privacy. Unless the owner voluntarily disclose it because he chose so, it will be difficult to get, unless you have a good and legitimate reason (I don't know what the procedure is in this case and if you can get it directly or have to get a court order first).
Search at the registry of the extension of the domain (often nic.xxx), or you can search with a lot of services, like for example whois.com, and ideally run it at the registrar where the domain is registered (you should find a link named "whois" somewhere). Expect most information to be private, but you can most of the time contact the owner (without getting his real email, though).
If all whois info is public, some services used to scan it and you could find all domains registered under that name. It is difficult now. If it's a "domainer", he may list the other domains for sale on a platform and you may find other domains he owns there (only what he chose to put up for sale on the same platform).
