if you go into the reseller control panel, and then click search for the domain..
then click hte account id, on the right of the pop up there is xxx.xxxx.com for server, the account is on that server and theregfore the domain also is
You can also go to the customer Control panel and go to "FTP USER". THe hostname shows the server as well. Sometimes you are already in the customer CP and it is a quicker way to obtain this information. 8)