To find out the owner of a website, there are two methods you can use :
1. Search a WHOIS database
Several sites specialize in finding information about website owners. Go to http://www.whois.net/ and type in the domain name you are looking for the owner.
2. Find the information in the imprint
Legally, it is mandatory to indicate on a website that hosts it, who is the owner and a contact. All you need to do is search for the link "Imprint" or "Legal Information" or an equivalent term on the relevant website.