Shake the jar for 10 seconds. Now hold the ends of the jar in both hands and slowly roll the jar. Look for small red spots stuck to the inside of the jar. These are varroa mites. Count the mites.
Beekeepers differ on how to interpret the results.
Fewer than 5 mites is a light infestation. Up to 10 mites is a moderate infestation. Over 10 is a serious infestation. Treatment depends on the number of mites, the time of year and the condition of the hive. Ten mites before the Spring buildup is not as serious as the same number in the Fall when bees are preparing for Winter.