It's test-spraying on a piece of cardboard or other material before I spray the model! Being an impatient individual as soon as I mix the paint and load the airbrush I get excited and want to shoot at the model. 8 out of 10 times this results in a coarse, sandpaper-like finish because the paint is too thick for airbrushing. However, after several poorly airbrushed models I've forced mysel into the habit on spraying on a small piece of black (for greater contrast) poster board to observe how the pattern forms. After getting into the habit of this airbrushing screw ups should be eliminated. The execisvely-thick spray coat that will waste the model is very distinct when shooting against a highly-constrating test surface. Also, when too much thinner is added the watery dilluted pattern will also present itself but 9/10 times whenever there's a problem it's a thick paint.
Modelers recommend thinning the paint to milk consistency but you'll never know if you have the right consistency or not-unless you test-shoot. Because of the molecular chemistry of the paint sometimes one drop of thinner makes the difference between a perfect pattern and that terrible coarse finish.