I think I've mentioned this before but the Porsche 917 high speed handling problems were solved at a practice session at Zeltweg by John Horsman's noticing that the pattern of smashed gnats on the car showed that there was no airflow over the tail; some sheet aluminum & pop rivets and the car went out again for another practice lap. The driver pulled into the pit and announced "Now it is a real racing car!" Thus, the 917K was born.
Careful observation of the pattern of dirt, oil streaks, rubber dust, or even bugs can tell you a lot about what is happening to the airflow.
Regards, Neil Tucson, AZ