Is there a reptile specialty store anywhere near you? They will probe your snake for a few bucks (or free, depending on the store). If done correctly, the snake won't even notice it being done.
Looking at the tails is just speculation. My 3- 2004 snakes all turned out to be male. One has the classic, male looking tail, one was kind of in between, but seemed male. My Okeetee though, I was sure was female based on tail observations. I was totally shocked when probing confirmed it was a male.
If you are curious because you want to eventually breed them, then spend a year or more aquiring new snakes and getting everything ready, you're going to very upset if you guessed wrong. I have orders in right now at Cornutopia for a female Crimson, and a female Okeetee hatchling. If I didn't have my snakes probed first, I'd have wound up with the pair of Crimsons I wanted, but 2 male Okeetees.
Good Luck,
Scott
