If they are getting concentrations for the dye in 1/10th of a ppm, then thye are not just checking to se if it looks red. They are doing a legitmate test. Maybe spectrospy on a specific wavelength, but that might be easy to get an over estimate. More than likely, they use a titration where they add another chemical to the diesel fuel that reacts with the dye. When you add enough of the indiacator to react with all the dye, then there is some indiacator left over. They can use a spectrophotometer to see how much indicator it takes for there to be some extra indiactor. You do some math to calculate how much indiacator was used to react to how much dye, and you come up with a number. Inorder to cheat a test like this, you would need to know exactly what kind of test is used to test for the dye. Then add enough indiacator to react with all the dye, then it appears to be undyes fuel. A test like this can detect a very small amount, and if they are arsholes, they can get you for fuel from a year ago. But this is alot more trouble than just buyin the road fuel, and the indicator might be harmfull for our trucks.