Apparently this is a way to tell, but is not fully reliable. I have also read you can tell by the jawline and also the color along the edge of the caudel, but again nothing really consistant. They are really cool to watch flare, i'm pretty sure i have at least one male and one female. The one i cant really tell is the smallest, probably youngest of the 3.
I put in a heater so i can keep it around 78 (only 74 now) and when i put it in, the largest of the 3 came over and flared at the heater. Its funny because everything they do seems to be in slow motion, but they do have a ton of personality. Lights are out for tonight, but i will try and take some video in the morning.