Cheese mites are invisible to the naked eye, but when clustered together look like very fine brownish dust. If you brush the "dust" off the rind of a wheel of cheese and a few hours later the "dust" has moved its location, you know the cheese has been infested by mites. The same type of mites are also attracted to flour and grains.
Mites tend to be present on the outside of hard cheeses, such as Cheddar. Usually, the mites can be brushed off the rind of the cheese without affecting the flavor of the cheese inside. Some people even think that mites give cheese more flavor - although this is not a commonly held belief.