From this thread.
[Mod edit: Fix up quote end tag --Mats]Coryman wrote:It is not always easy to determine which of the two species you have just from a simple view, as both species can and do vary in colour pattern.
According to the description C. duplicareus has serrations on the posterior edge of the pectoral fin spine, whereas C. adolfoi does not. This of course is not easy to see in a normal aquarium. The best way to see if your fish have these serrations would be to place them into a shallow, preferably white, container and shine a light up from underneath. Then with the aid of a magnifying glass you should be able to see if there are any serrations or not.
As a general rule C. dupilcareus tends to be a little stockier than C. adolfoi and the black band is usually a little broader.