Your igs are what, about a year old? Trust me, around 18 mos or sooner (if you've been doing everything right) your igs will let you know if they're boys or not! It is very easy to find out- when they are pooping, try to get a good look at the vent on the last "push". Males will pop the hemipenes out of their bodies while pooping. The hemipenes look like pink fingers.
Of course, coloring, body shape, femoral pores, and breeding behavior will tell you their sex, too! Probing is very unpleasant for your iguanas, and potentially dangerous. If your vet has any concern for the iguanas, he will probably want to at least sedate them before performing this uncomfortable procedure. This sedation in itself can be dangerous for iguanas. Please do not have them probed.
As for breeding them... you only need to look at your local PetCo, Today's Pet, or reptile show to see that your competitors are charging an average of $10/iguana. Sometimes they even have a 2 for 1 sale. Let's say you have a clutch of 30, and most survive their first few weeks... are you really going to be able to offset your breeding cost with the measly $300 you would make??
No. Vet bills, x-rays, food and housing will total much more than that for your breeding pair and babies... if you have no qualms with sentencing lots of little iglets to death and disease, then at least consider that breeding your igs will not make you a cent, and will probably land you in debt. If it's baby igs you want, just search around the adoption sites and rescues- they aren't as common as juveniles, but you'll find them homeless, too.
I know you will do the right thing...
-Erin K.