How to tell male from female?

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Male

 

A male will have two lumps under the tail behind the vent.
Males will also have larger femoral glands (on the inner thighs) than a female.
A male's head will look more like an arrowhead shape, where a female is a bit rounder behind the ears.
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A male's tail will be thicker at the base.
Males will turn their beards black and bob their head up and down in the presence of a female or another male.
Sometimes only to say
"This is my territory"
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Female

 

Females have several features that differ from males, However, One single bump under the tail is the most certain way to tell the difference.
Females will sometimes wave their arms in large circles. This is to get the attention of a male. 
Females tend to have smaller tails at the base, than the males, and females head is not as sharp of an "arrowhead" shape.