A good indication is 1) where their name is registered. Sure it isn't fool proof, but more often than not, the domain reseller's name is registered with the registrar they resell for. Imperfect science, I know.
Also check the TOS for the registrar. Usually there is...
1. AGREEMENT. In this Registration Agreement ("Agreement") "you" and "your" refer to each customer, "we", us" and "our" refer to TUCOWS.com Inc. and "Services" refers to the domain name registration provided by us as offered through...