Shake a frame from the edge of the brood nest into a plastic dish pan. Be careful not to shake the queen. Scoop 300 bees (1/2 cup) from the brood frame into a wide mouth mason jar. Cap with cover fitted with 1/8 inch hardware cloth. Add 2 tablespoons powdered sugar. Roll the jar gently for exactly one minute, set in the shade for exactly two minutes, and shake vigorously over a light colored surface or bowl for exactly one minute (the back of your dishpan works well). Spray with water to dissolve sugar and make mites show up. Count mites. Infection rate is the number counted divided by 3.
Learn how to do a sugar roll test on your own through this article, and learn the thresholds to keep an eye on to decide whether to treat.