We want to split the population according to sex and measure certain body parameters separately. As both sexes have antlers unlike other ungulates, it is quite complicated...
You need to have two important things:
1- the complete description of m& f in of that herd.
2- a full coverage picture of all animals.
It is very easy to tell the gender, as long as you got a sharp image of adult individuals. Males got antlers that split at the base, with forward pointing horns. This feature is lacking among the females. Secondary characteristics to look for are that males got a beard on their throat, the shape of the head differs slightly, and not to mention that even though the penis is retracted one might see miscoloured fur on that location.
I agree with @Anders. Male reindeers have antlers that split at the base, with forward pointing horns. Whereas female reindeers lack this feature . . .