Pretty much all the buses come with optional roof height. The way to tell by just looking at them, you take a look at the bus from the side profile at the roof line, if you look at enough of them, you'll notice 3 different style roof lines. The lowest ones (and unfortunately the most common) are the dropped roof, you can see the front roof cowl and the rest of the roof drops about 3" all the way back, these are "Generally" ABOUT 6' in height, if the roof goes straight back from the cowl (just above the front windshield), then it's likely to be around 6-3, and if you see the roof raise up from the cowl all the way back, those are the 6-6 roof lines...
Dropped Roof
Straight Roof
Raised Roof