Most OBs these days are line bred, which sometimes tends to blur gender rules.
"Norma"l male peacocks and Haplochromines (in general) will be more colorful, and sport very prominent egg spots, or streaks on the anal fin. (like the Nimbochromis below right)
Females on the other hand are more drab, and if they sport egg spots at all, tend to be washed out.
Line breeding, random aquarium crossing, and intentional hybridizing tends only to confuse.
Its only when spawning occurs, that differences appear, as opposed to pure breeds, like the Shuja (Ctenochromis) horei pair spawning below, where male female differences are obvious
Agree with the above. An educated guess would tell me female. How large the fish is and tankmates would help guess better however. I have many OBs, and there will always be ONE dominant one, so if you have others in the tank it could be an inferior male. As stated above given the hybrid aspect some of the normal rules don't apply. I have a few females that are more spectacular than some males, however that is not the norm, most of the females have blotches and a greyish/white backing. There are so many variations of OBs that it is very difficult.