Not what I expected when I clicked this thread, but alright. Well, I can think of a few points...
- The overall body is much slimmer than most other lizardmen. Even those that are smaller generally tend to be more bulky, with only a few exceptions. The shape of the body is a bit more towards hourglass with a thinner waist and bigger hips than a V-shape, which is pretty typical in male vs female design.
- The feet and stance of them is very pointed, giving an impression of high heels, which is another staple of female design in other factions (the questionable of choice of wearing stiletto heels to battle aside...). The other lizardmen typically have their feet more horizontally aligned with the ground.
- There is a slightly pronounced bust, and the area is "strategically" covered. It's not enough to say that the artist was going for the character having actual breasts or give the impression thereof, but still. The other lizardmen are generally either bare-chested or wear armor that covers most of the torso.
Overall, it could go either way because it's not obvious that any of these points were intended to give a female impression, but in context of the faction, Venomage is certainly the closest to having a design that is in line with that of other female characters in the game. Of course, given that they are not humans, or even humanoid strictly speaking, but rather anthropomorphised lizards, there's nothing to say that, I dunno, Aox or Jareg couldn't be female based on their design alone.