It can be difficult, I have a pair in amplexus as I type (woohoo!). My female is larger, but I havn't had enough to know if this is sexual dimorphism, or if she's just the oldest. I've had her as an adult for 7 years. Females will also have a larger cloaca, which will balloon even moreso when she's ready to mate. Males will click on occasion; I"ve never heard my female click before.

Ian