popping is safer for the snake and seeing the organs visually lets you know for a fact what the snakes sex is.
I have seen way more people probe snakes wrong and come up with the wrong sex...

Popping takes practice, it's the only way you learn, and boy is it satisfying to know you won't get taken purchasing another snake of the wrong sex.

Ralph davis's pro popping video on youtube is an excellent video showing technic
http://www.youtube.com/watch?v=BPmIpc_gP9I

Good luck learning to sex either way