I have a pilot friend who races Long EZ airplanes. His advise on vortex generators was to paint used engine oil on the surface you would like to add them to. Then go drive on the highway for a few miles. The air washing over the oiled panel will define where the air flows, and thus where you need to put the generators to create separation for cleaner aero flow. It does work but its messy....
This is not his EZ, but one like his.
