I have very little faith in popping mostly because I suck at it. A snake can be obviously male when popped but in my opinion could still be a male if it was not done correctly. When I buy a snake in person unless it is obviously a male I have them probed and I witness the probing. If the seller is unwilling to probe I do not buy the animal. On my own stock I do not pop. I am much more comfortable getting some help and probing the animal myself.
You should be taught to do either technique by someone that is experienced. You do not want to learn to do by trial and error because the animal could be hurt.