The suitors know they have no chance of surviving when

O they realize the beggar is Odysseus returned.
O Odysseus kills Antinous with his bow and arrow.
O they see the shape of Athena's shield above the hall.
O Telemachus arms Eumaeus and the cowherd.

The suitors know they have no chance of surviving when they realize the beggar is Odysseus returned and they see the shape of Athena's shield above the hall.