Check out this helpful article from the ABA Litigation Section.
I personally once called someone to see if they would review a case. His assistant seemed surprised at my request. I then looked him up on the Medical Board site and found that he was under Board Order for abusing anesthesia drugs and had once been found passed out in an OR after self-administering drugs. Lesson: Always check licensure status before making the first contact!
Sometimes we get so enamored with our experts and their helpful opinions that we look at them as opposing counsel or the jury would. A good practice would be to spend as much time researching your own experts as you do researching those retained by the opposing side. While these aren't the most "fun" topics to discuss with your retained experts, finding out this information well in advance of a designation deadline could save you time and embarassment later in your case.
My thanks to Francisco Ramos, Jr. for his great article.