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Cut a bunch of 3" long pieces of yarn, tape them to the surface of your car about 6" apart each way, then drive down the road at various speeds while you, a friend, or a video camera observes the behavior of the yarn in the air flow. In an attached airflow, they will all point in about the same direction with very little jumping around. In unattached or recirculating airflow, the yarn will be jumping around in all directions. Google "yarn tuft testing" to see photos and videos of it.
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