how do you tell if a iguana is a male or a female?

Answer 1

A MALE HAS SUCH A THING CALLED A 'PENIS'

Answer 2

If it is over 6 months old, the easiest way is too look at the femoral pores on the inside of the back legs. Males will have large pores, and females will be very small. Here are some pix:

http://homepage.ntlworld.com/steve.woodward/sexing.html