a person who is especially competent to pass critical judgments about cocks (as in dicks), particularly about size, length, girth, shape, "angle of the dangle", circumcision, or lack thereof.
a connoisseur of cocks.
A prostitute could be considered a cockoisseur since her main job function involves cocks.
A urologist could also be considered a cockoisseur.
a person who has special skills, knowledge, and/or experience in the particular field of SEX
An escort (or prostitute) if they are very good at their job could be considered a SEXPERT.