How long ago?
If they are living, you can check with the AMA to see if their license is current. That would at least let you know what state they are in.
Now, if you mean a long long time ago, there are other ways. I needed to track down a doc who quit practicing in the 70's and died in the 80's. I looked back through newspaper archives, got the name of his last office manager, and got a hold of her. She was able to put me in touch with the doc's son.