Tuft testing is one of the easiest, most effective, most useful forms of testing; you can trial things like separation edges, mirror changes, fences, vortex generators, air curtains, etc. and see what
actually happens to the flow (which is worth an infinite number of guesses). That
is a measurement: the tufts show flow direction and, as you learn to read them better, turbulence and speed. Most importantly, the tufts will show you exactly where on the body flow is attached and where it is separated.
An example of useful tuft testing: you asked about your muffler. Tape tufts to it, station a friend on the side of the road down low with a camera and take pictures as you drive by. Is the flow attached or separated? If it's attached, what do you think a plastic panel would "fix" exactly? If it's separated, throw a piece of cardboard on it temporarily and see if it's better, worse, or the same. Use your head rather than throwing up your hands and going, "Well, looks like all I can do is guess." I used to be that way. It's a giant waste of time.
For pressure testing, you can find digital manometers on Amazon for $50 or less;
I bought this one, which came with two pitot tubes at the time but
you can buy one separately. Disks are easy to make at home with metal bar stock and small brass tubing, which you can find at hardware or hobby stores.
Julian's book walks you through the process. He was also nice enough to collect explanations of various test techniques in a
single thread here.
This quote--from a 1963 paper on aerodynamics and body design by two GM stylists--is as true today as it was then:
"Technical apathy is often a far greater deterrent to the acquisition and application of vehicle aerodynamic data than any excessive expenditure of time or money" (
Kelly, Kent and Harry Holcombe, "Aerodynamics for Body Engineers," SAE 640050).