Discussion is the best thing that happens here, so no worries there.

Prior to the battle...no, no-one COULD have known. But the ground troops would have figured it out VERY quickly, and then Luke's announcement to the same effect is made for the audience...very early on in the battle....and yet for the entire rest of the sequence we get shots of rebel troops firing away (aiming horizontally over the ground as well). That's often bothered me. I always felt they needed something to be shooting at once they figure out that their weapons are ineffective against the armour advancing on them....and yet keep shooting.