I did yarn testing of my VW. I put on some heavy wire masts on the back and tied a flag every 3". Once you see where the wind is not turbulent you can use that point for an "anti spoiler" What surprised me is how quickly air went from being fairly organized to a frenzy. It would only stay together a few inches past the edge of the body.
Here is an over the shoulder pic of it a few inches behind the back window.
I used a section of cattle fencing. to hold the tufts. I know a guy who uses a fiberglass wand (fishing pole) with a bit of yarn on it. He can hold it where he wants to test.