Tail size is definitely an indicator, but at 2 years of age I'm not sure if you would be able to tell. You might have success by looking at the cloaca. If it goes beyond the edge of the shell, then it's likely a male.

I have a spiny softshell and I'm also hoping it's a male. They grow fast huh? Mine is a yearling and is about 4 inches. You have pics? I would love to see that 125 setup!