Well, now a days the only sure way is to research the history..... I have names from 1997 or '99 that technically dropped but never really. The best way is to research the email addresses over several years, as well as name servers and other details. But domaintools.com is really the best to utilize. Wish they had an affiliate code or something:yes:.
Though your question is somewhat ambiguous for professionals because a name may be available to the public, (technically), but not really. Majority will be and is held by the registrars.