ive caught many, many, many adult commmon snappers and florida snappers, and everytime ive noticed males have bigger bulkier heads in comparison to body size. ive also noticed that a young (6in) turtle with a large head, usually grows into a BIG turtle. its kinda like a puppy's feet. im pretty sure its the same with allys, at least from the hundreds that ive seen. though, i wouldnt rely on it as an accurate way of determining sex.