Dorsal fin might give you a better idea. You'll definately know for sure when they hit the 6 inch mark though. The rosey ventral coloration will start to show on females at around that size if they are healthy. From what I've read, any roseyness on the ventral side of their body means they are definitely female. I can see the dorsal spots on one of them in the picture, I know that my female has spots on her dorsal fin, but they didn't become clearly spots until she was almost 7 inches long.
