[TJ Mason]

A good indicator is where your drawing hand goes after release. If it flaps outwards ("the Hello release", as I've heard it called), then you're probably using arm muscles rather than back muscles. If the hand remains at the anchor point then you're probably not using back tension -- at least not properly. The hand should come several inches backwards and behind the neck, and perhaps downwards.

You can get perfectly good results without using back tension, but chances are that your consistency will then suffer in the later parts of a long round.