Find out if he has trained under a JKD instructor. Not a seminar or two or read Bruce Lee's books, but actually trained. Then do your research. The internet is a pain sometimes, but its a great tool for fiding out if an instructor is really legit.
Even if the instructor doesn't have a website, it shouldn't take long to state what his lineage is. Who is he certified under? Here is an example of an instructor showing "typical" high level lineage in JKD and associated arts http://www.pamausa.com/Pages/rick.shtml