Attaching yarns is a way to see what the air is actually doing..
Another trick I saw being done by one of the Formula Ford (tiny little
English ford motors in 800lb chassis racers in SCCA racing) guys was:  He
had a little oil can full of red oil of some kind.  He'd go around his
driver's car and "ploop" these little dabs of oil out in areas he wanted to
consider.  The guy would go out and turn a lap and come back into the pits,
where the mechanic would see where the oil had been blown...Some funny
surprises and some strange 'eddies' on the racer...The next race, they'd
come back with some winglets, bumps, etc etc..and do the  oil thing again
during practice.  This guy would consider the length of the "oil smear",
it's direction, etc.
  Seemed to work for him, he improved lap times consistently,  with
everything else being carefully specified as "no change" by the rules
  Another aero aid that people misunderstand is called a 'splitter'...You
will see them on fairly sophisticated race cars where they fool the air into
thinking the car is longer than it actually is.  These splitters are like a
shelf that sticks out near the ground on the nose of the car.  The car
pushes into the air, splitting the flow into a lot that goes over and some
under.  The splitter "collects" a big relatively static high pressure "pile"
of air which stays on top of the splitter acting like a part of the
car...making it act longer, hence having a better shape..
  I saw a Viper with a piece of CDX 3/4" plywood stuck on the front with
corner braces...speaking of your "Home Depot race team"  Worked for him, I
guess.
  Don Hanson