A good place to start might be to use whois to check the IP with arin.net: whois -h whois.arin.net xxx.xxx.xxx.xxx. That won't necessarily give you the final answer though. The best way would simply to ask your client to check who his payments are being made to.

-sauoq
"My two cents aren't worth a dime.";

This doesn't work, either. In my case, I get some company I've never heard of, which is most likely whoever hosts the company that hosts me. I think your best chance is to do as others have suggested - ask the company to check who they're paying. Low-tech, but if it works...

This doesn't work, either. In my case, I get some company I've never heard of, which is most likely whoever hosts the company that hosts me.

Right. Like I said, this is a starting point and won't necessarily give the final answer. But, you can find out who is responsible for the IP and work your way down by contacting them. (They might even have their own whois server.) I do think the guy's client should have a look at his records to see who he is paying... but, there's always the possibility he paid for a year up front 6 months ago and has since lost the info, closed his bank account, changed banks, paid with a money order initially, or whatever.

-sauoq
"My two cents aren't worth a dime.";

Look at the DNS servers, those are almost certainly provided by the host.

That's simply not true. DNS is very often provided by the registrar or the host's host (or the host's host's host, etc.) in the case of resellers. And it could also be provided by a third party contracted by the host.

-sauoq
"My two cents aren't worth a dime.";

Ok, then "almost certainly" is over-stated. But it's certainly one of the easiest places to look and especially for the the larger ISPs (which are statistically more likely to be the host) it would work.