Purely by sight driving down the road. The bumpers are the main way to tell the difference. All GM 6.5, except the later 2000- 2001+, have the 2 large holes in the front bumper. They are needed for proper cooling.
Gas powered trucks, Tahoe, Vans, Subs, ect do not have those holes.
Hard to ID strictly by exhaust. Exhaust could be aftermarket. Or a bad running gasser will carbon up the end of the pipe like a diesel.
I have seen very few 6.5 2 door Tahoes. Not a vehicle I see every day, or every year for that matter.
Judging by the lack of responses to this post. Not many members here here have them. I have seen a few posts about 6.5 Tahoes over the years but not very many.
What is even more rare is a 6.5 diesel Tahoe with the Barn Doors in the back. I have never seen one. But they do exist.