Sexing adult snake

I have a 6 foot plus red tail and was wanting to try my hand at breeding them and was wondering what is the easiest way to sex a full grown snake.

Probing would be the best I think, but only by someone who's experienced.

x2, Its the best way to make sure exactly what the sex is. But only by someone who knows how to do it. The snake could be harmed if done wrong.

Must be probed ... You really can't safely pop a snake of that size well in my opinion.