This is a good one, take a nav light bulb from you spare lamp store (back of the flight deck)

Get a nav light bulb (P/N MS 25338-7079) commonly know as a 904

With at least one pack on. Turn on the Gasper fan and close all other flight deck outlets.

Depending on who is going to do it Capt or Co Pilot, open the relevant gasper outlet, Turn the outlet straight up, pointing at the sidewall almost. Place the bulb close to and above the outlet in the airflow, gently let go. Should float in mid air about 6" from the outlet.

A nice example of the old Venturi stuff we got in training.

If the bulb is displaced the turbulence is light - moderate (Joke !)