He doesn't need to see it because he already knows their tactics. The line before is...
Major Mitchell: It's settling directly over us.
Which is confirmed by the 2D radar plot. Gen. Grey then announces...
Grey: They're preparing to fire their primary weapon!
Which is what they do when they settle over something.
Interestingly, the film's official novelisation (based on an earlier version of the script) has a different take on it. Grey can see a 3D radar plot of the ship, with sufficient detail to make out the enormous doors.
Connie had come into the war room to see if there was anything she
could do. Standing behind Grey, she watched one of the radar screens
showing a three-dimensional display of the huge city destroyer.
Because some of the base’s primary radar receptors had already been
destroyed, the image on the screen was incomplete, blinking on and off
like a ghost. She felt a long shiver run through her legs up to her
scalp when someone reported that the thing was directly overhead, then
pointed out to general Grey some aspect of the torn, indistinct image.
Once he understood what the man was showing him Grey snatched up the
microphone and spoke to the remaining pilots.
“Attention! They’re opening the bottom doors and getting ready to fire
the big gun. Somebody get down there and knock that thing out before
they can use it!!”