In the old days you would have to get the court to qualify your doctor as an "expert" in order to allow him or her to testify during a trial.
You would have to present the doctor's qualifications and then formally asked the court "Your honor, I ask that Dr. Jones be qualified as an expert before this court."
That formality has pretty much been done away with in New York state courts. Instead, you still have to present the doctor's qualifications to establish that he has a basis upon which to render opinions and conclusions.
However, it is now the jury's decision whether to believe the doctors credibility and qualifications. The jury is allowed to weigh each of those factors to determine whether to accept or reject the expert's testimony.