Doug is right on the money here.

I've popped (or attempted to pop) hundreds of snakes of various species, and mussurana have been the most difficult to sex in my opinion. I can't seem to get the males to pop. I've had a lot more success probing them. As for probe depths, my females have probed fairly shallow (3-6 scales) and the males seem to be more like 10-12 scales.

The good news is that they grow quickly, and the bulging on the males becomes more and more conspicuous as they grow.

My $.02