When you hold JJ, the flop will contain at least one higher card the majority of the time. That's w key reason why putting in a decent-sized raise before the flop is important; to try to get mediocre hands out of the hand e.g. Ax, Kx or Qx suited that a fair number of players will play if you let them limp in.
Then if an A comes on the flop, you know you don't have one in your hand, but the opponents don't. If you bet out like you have a good hand, someone who stayed in with Ax may well fold.