I'm in the same boat as you. Do they expand their neck and body when they croak? In my head I have it that that is how you know it's a male. I have 3 whites and I think that I have 2 girls and a boy. My reasoning is, when I play the frog calling from YouTube only Olive will try to mimic the sound by filling his neck and body with air. However, Shrek and Skitter have both made short noises that almost sound like a parrot squacking. Just like your frogs it's usually when they get stepped on. I've also noticed it when they get upset or when they get excited about feeding time they do it. However, Olive will only make a noise when I play the YouTube clip LOL!

When I first got Shrek, he had a tank mate named Fiona and I always thought she was a boy because I could walk past the room and hear her croaking, but when I would try to catch her she would stop!

I wish I knew at what age or size we'd find out for sure.