How is drag / Cd quantified in a wind tunnel?
 

Are they just about smoke testing, or can they actually measure changing force on a vehicle (through sensors/pads in the floor?) as body elements are tweaked/streamlined?

I have no clue but I'd have to assume that they have sensors that measure the overall drag force.

The frontal area gets measured and then the Cd is derived by dividing the total drag by the frontal area to get the Cd.

Good question!

Force gauges ;)

One setup involves the subject to be supported by a pole (stinger), where deflection/force can be measured. I'm sure big operations have much more sophisticated methods.

Smoke testing shows where separation occurs, so changes can be made ;) It also shows if certain features are doing what they are supposed to (such as duct work).