First thing you should look for in chameleon is the color, in most species of chameleon the male is more colorful. Another thing you should look for is the extra digit on the hind legs of the chameleon (as shown in the picture below). Also when a male chameleon hits a certain age he will expel sperm plugs which I will put a picture of below and in some cases you will see hanging out of the poor guy. If you have a female chameleon you should seriously look into setting up a laying bin for her very soon because she can become egg bound which is a serious problem.