Males show their sex about 2 weeks earlier than females of the same age... so if you have 3 or 4 plants of the same strain, all started at the same time and some of them are developing really quickly.. chances are you have a male.

Males are balls on little sticks.. females are little sticks on balls protruding from the nook of the branch. Males will develope faster. and Females are easy to spot once the white hairs begin to show

Cheers,
Bob.