If a company is using domain name server in resemblence with their domain name, its really easy to find-out. Eitherwise search the who-is of the domain name used for DNS entries, maybe you get a clue from there.
Often with shared hosting you just have to forward resolve the domain to the IP, then find out the reverse of that. The hostname will usually be related to the host in some fashion. Failing that you can try just entering the IP into your web browser and see what you get, or whois, but most hosts are too small to have their own IP allocation, and most clients put their own info on their domain registration, so this won't usually get you anywhere.
Ichiro, there really is no way to hide the host. Even if you play around with DNS its simple enough to whois the IP address(es) of the shared machine and find either the host in question or the company that reseller deals with. (Keep in mind in this case when I say host, I mean whoever the IP addresses are delegated to) I've also seen a lot of companies default to redirecting to their websites if you put an ip address into a browser for shared machines.