Lectures do not mean much. Many of these may be drug company sponsored, invitations by friends, etc.
Here are a few things you can look for in order to get some idea:
Look up number of published papers on pubmed as well as textbooks, etc.
Academic title. In most institutions, it requires a significant amount of academic work to be promoted to the associate professor level and much more to the professor level.
Talk to the people at your medical school that are in the same subspecialty.
Lastly, remember that being well know does not mean they are good teachers.