You can find some info here: Phrynosoma.org

It's pretty weak actually and needs some revamping. I've been meaning to get around to this actually.
In general, the males have the bulges just aft of the vent, whereas the females will generally taper off rather quickly.
Some species have enlarged post-anal scales (Usually just one large scale on each side) that help indicate a male.
Enlarged femoral pores along the underside of the rear legs is also an indicator of a male.
Hope this helps some.
MarkB
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Phrynosoma.Org
Phrynosoma.Com