The problem with external features is that unless you have a known snake of the opposite sex, you won't be able to compare really.
The things like slightly smaller head or stubbier tails are a)not very definitive and b) meaningless unless you can sit there and compare directly.
It's like trying to figure out the sex of a baby without looking. Yeah, you might be able to go on hair length, but if you didn't have another baby that you knew was male or was female, how would you know what long was? And even then, maybe the boy's hair just grew faster.
Just have your vet probe the next time you go in. An occassional check up isn't a bad thing.