If you want to tell by the tail shape - and you're not an expert, an easy guide is to count all of the scales (on a shed skin) from the vent to the tail tip.
< 130 - probably female
130-140 - ??
> 140 - probably male
This is just a guide, and shouldn't be used for breeding/hosuing snakes together.
I beleive experts can pop/probe snakes a young age, but obviously it's easier, and safer, to do this when they're big. This is something you shouldn't do yourself !
HTH