"Check the space between the pectoral fins (the two narrow ones below the gills) and the anal fin (the lower fin just in front of the tail). If the space is almost horizontal between the fins, this is probably a male. If the space is angled down a bit, this is probably a female.
Check the opening at the bottom of the fish. If it is small and narrow, it is probably a male. If it is bigger and wider, it is probably a female.
If you are lucky and the fish is in breeding form, you will see a small tube extending from the opening. The males have a small, pointy tube, and the females have a rounder tube (for laying eggs). This is the most accurate way to determine gender. Females also get a little round belly when they are full of eggs, though males can get a temporary belly if they've just gorged."
evo copy paste sa net-a ali sve u svemu ... određuje se teško