It's often one of the most difficult tasks for an umpire: discerning whether a ball has clipped the bat or part of the player's protective equipment or clothing on its way to the wicketkeeper. The Snickometer, although not used in adjudicating decisions, is a useful TV tool which tracks the ball's passage past the batsman by picking up sounds from pitch and stump microphones. The usual rule of thumb is that no sound (top picture) means that the ball has failed to touch the bat, while a 'short' noise (bottom picture) probably means the ball has hit the bat.
No noise
No contact made here ..... and no problem for the umpire with this 'not out' decision.