hi mate think that might have been me.
ill try and explain.
First thing to look for is the overall shape. IE females are a more rounded shape. Males are slimmer and from a young age males will normally show the red at the back of the head. The best way is to view them from above. The head on the male will look longer and you will start to see a ridge develop up the snout just before they start to grow bristles. This normally starts when they are coming up to the 2" mark. observation is the key and comes with experience when you breed and rear them constantly. i normally pick ten out from each of my breeding groups. Ten males from one and females from the other and rear them on to adults before selling them on.
Hope this helps