You’ll need about 300 bees from various parts of the colony (entrance bees, frame bees, bees on the super frames), a jar with a lid and starter fluid. Fill the jar with the bees and spray them with starter fluid. This kills them but also knocks out the mites.  Draw a line with a marker down the edge of the jar from top to bottom. This is your starting point. You’ll shake the jar vigorously round and round to get the mites to fall off and stick to the side of the jar.  Rotate the jar in front of you to count the number of mites.  If you have more than 3 or 4 mites you should treat your colony.  The line tells you your start point and not to count beyond that so you don’t double count.

This simple test gives you a baseline to determine the number of mites in your colony (approximately, counting them all would be difficult to say the least).  You want to avoid treating hives that don’t need it to avoid contraindication with regard to resistance to a miticide.