After awhile, you can tell the difference w/o having to try, but at first they appear similar; and even now, sometimes I have to do a double-take depending on the angle, just to be sure. The quickest way to tell the difference between them is to look at the door handles: a D1 has the flat-lift-plate door handles, whereas the D2 has a grabbable handle and a thumb-button. From the rear, the D1 taillights are low, below where the sides begin to taper up to the roof; on a D2, the lights are moved up to eye-level on the pillars. Even if you can only spot the roof, the way you can tell the difference is the alpines: a D1 has a rubber gasket visible around the outside of the window, a D2 window is flush with the roof, you don't see the gasket surrounding the window perimeter. From directly in front, the early D2 looks a lot like the D1, but you can still spot the door handles sticking out from the sides of the D2. The later/current Discovery has the projector-spot headlights (angel-eye lights are only on BMWs, not on Rovers).
A D1 is a bit more mechanical; the D2 is bit more refined, but has more electronics incorporated into it. Maintenence is important w/ any British vehicle.
IMHO, FWIW......
-L