you can't most of the time.
only when the (sexually mature) male is showing a
nuchal hump (a bump on the head just above the eyes) and most of the time
even experts can not tell by this method, or when breeding tubes are seen.
Females have a blunt tube called the ovipositor that is used to lay her eggs & males have a narrow tube called the papillae that is used to fertilize the eggs.