To find out who your domain provider is, use a website like DomainTools or the WHOIS Lookup provided by ICANN. Information like the Registrar or Name Server can be indicators of your domain's provider.
More tech-savvy users can use their computer's Terminal program to look up the domain's name server by typing in dig -t NS [your domain]. For example: dig -t NS bitly.is. A domain's name server can be a good indicator of the domain provider, but it depends on the provider's naming methods.