Look up a set on any seller's website and just look at where the stub axle is centered in the spindle casting. It's pretty obvious and you'll be able to tell from looking at the back of yours while they're still on the truck, no biggie.
OK, since I'm such a nice guy and telling you how easy it is, here's a set of Belltech drop spindles for a GMT400 dually.
A stock spindle will have the axle lower on the casting, 3" lower, for that model.
If it helps give a little perspective, here's a pic comparing half ton stock spindles to 2" drop spindles:
Richard