The best way is to tell by size when they're fully grown. My males don't always have a really dark throat. It comes and goes depending on the season. I would say calling is the best indicator...but I had 1 male that never called, and another male that didn't start making noise until recently (sounds kind of like a whimpering puppy instead of a normal chirp.) Although if they do call, you're guaranteed a male. Here is a thread with some pics I've shared in the past and lots of tips on sexing - http://www.frogforum.net/showthread....ight=grey+tree