Now I'm no expert at all on this, my first blackthroat was obviously male because he insisted on getting his bits out all the time in my presence. I took it as a dominance thing. Hemipenes on a blackthroat are pretty big, look for a flowery bit on the end whereas Hemiclitori are smooth and more tube like. Females in general tend to be a bit more flighty than males.
Finally you've got the bulges at the base of the tail, male bumps I believe are less smooth than a females, the males standing out more but like I say I'm not an expert unless they like get em out!