Perhaps you're not asking the right questions. How about, "What is the best way to test a new aeromod?" or, "Is dealing with the high pressure pocket in front of my windshield worth my time/money?"
If you're looking to test an improvement, mist or smoke is probably not the best idea. A deflector at the base of the windshield (I assume that's what you're planning) is not going to do much, so even the almighty coast-down test might not even show results. You could try tuft testing or coast-down, but don't expect much- even from the best executed windshield deflector. You might not even see an improvement at the pump.
Try a grille block, air dam and smooth wheel covers instead- they will actually help your FE!
H U M A N . P O W E R