De acordo com a novela Júnior ROTJ , parece que foi porque a Frota Rebelde viu a explosão do gerador de escudo na superfície do planeta:
Han Solo ran out of the Imperial bunker shouting, “Move! Move! Move!” Chewbacca and the other Rebels ran away and dived for cover. A moment later, the bunker exploded, followed by the turbine generator and the reactor core. The generator tower was suddenly consumed by a series of explosions, and the enormous dish-shaped shield projector array came crashing down to the ground.
The destruction of the moon-based Imperial outpost was immediately acknowledged by the space cruiser New Home’s Mon Calmari crew. Admiral Ackbar announced, “The shield is down! Commence attack on the Death Star’s main reactor.”
Curiosamente, a romanização oficial parece entrar em conflito com isso; que o almirante Ackbar foi capaz de ver o escudo em seu scanner e que quando o escudo caiu, houve uma confirmação do scanner:
On the Rebel command ship bridge, smoke and shouts filled the air.
Ackbar reached Calrissian on the comlink. ‘The jamming has stopped. We have a reading on the shield.’
... [Shortly afterwards]
A captain ran up to Admiral Ackbar, his voice tremulous. ‘Sir, the shield around the Death Star has lost its power.’
Ackbar looked at the viewscreen; the electronically generated web was gone. The moon, and the Death Star, now floated in black, empty, unprotected space.