Sometimes you can feel the rotation between parts of the axle that shouldn't be there. It can be confusing, since there is normally a good bit of rotation in the transaxle itself (so the wheel will move back and forth quite a bit normally - when that corner is jacked up, of course). But if you're feeling movement between any of the CV axle elements (shafts, cv joints), it's a bad axle. But as BajaUtah said, it's most often diagnosed by the crunchy noise the axle makes turning a corner.
Oh, and be careful about cheap Chinese axles. You'll be replacing them again VERY soon in most cases. Better to get a used axle, IMHO - or a quality rebuilt axle (I've had good luck with Cardone rebuilds).