Whois.sc/*domain*
In the *domain* bit we will use this forum as an example.
www.whois.sc/v7n.com
And looking on there you can find IP Address, and IP Location.
IP Location helps a lot, as you can see - "IP Location: United States - California - Los Angeles - Serverbeach" Severbeach is V7's dedicated host.
However, Name Servers help as well, as you can see V7's NS's are:
NS1.D7HOST.COM
And NS2.D7HOST.COM
So that's pretty simple, IP Location, and Name Servers.
But, in some cases, the above will not tell you the host, and that's where further research comes into hand, but I'm not going to go in depth.