Like a lot of central and south american cichlids, the males do tend to have longer finnage than the females of the same age. But also like most neotropical cichlids, it's not an 100% accurate way to tell. Plus the fish have to be on the larger side before signs of the fins start showing up. I saw a group of keyholes in a lfs at about 3-3.5 inches (just under 10cm) that looked like I could pick out males and females by the fins.
But as I said, like most cichlids, individuals vary. I have seem female dwarf acaras and convicts with longer fins than the males before. Not a normal state of affairs but can happen, which is why I say it's not 100% accurate.
^100% agree. Would also like to state that in my experience when the males anal & dorsal fins started to grow longer they also came together just slightly, whereas the females fins (although 'pointy' as well) grew almost straight back.
Have to re-iterate this was on mature specimens... I had 4 in total only 2 I was certain of the sex (because they spawned).
Here is a pic of my old pair... Female is in foreground to the left (notice her fins are pointy as well - which re-affirms what dwarfpike is saying), male is background and to the right - notice how you can see that his dorsal fin grows down and back so that it extends past the caudal fin - but does so lower than the females - practically overlapping the tailfin... you can't see it in the photo but the anal fin does the same thing...