The Legend of Korra

For the Air Bending part: If I remember correctly, it was mentioned that you are taken away from your parents to become an Air Nomad. So by that information we can assume that they determine who is an Air Bender before making them an Air Nomad.

 Spoiler

As for the Metalbending Police....maybe not all of them are that good, however they also don't even Earthbend at all. They throw away half of their abilities. Toph was so great because she combined the two. Lin even kicked some major ass when combining the two in the last episode, and the only reason she lost was because she made that stupid blunder with the Lightning Rods. She was winning before then.

The reason she lifts her foot to do Sensing is because she has to open the metal casing first. It's probably a ton easier to do it when the foot is in the air rather than when your foot is already plastered into the ground.

Characters like the opposite Pro-Bending team likely would have been important characters had they not lost their bending so quickly. Even though that guy was a jackass (The leader of the winners), he was still a skilled bender and would likely have been a powerful ally should he have kept his bending. There's other people like that. Most characters who lost their bending never had time to develop into major characters.

Hmm, I would think they took them away from their already Air Nomad parents, as in they only take people who are already in their tribe to begin with.

Anyway, did Toph learn Earthbending completely from the Badgermoles, or did she just get better at it through her interaction with them?