Another way to estimate the varroa in your hives is to catch the bees in a
jar with a screen lid (8 mesh: 8 wires per inch) and pour a tablespoonful
of powdered sugar in with the bees.  The mites let go, and you can shake
them out onto a piece of paper.  You wait and shake several times, and then
let the bees go again.  If you have trouble counting the mites, you can put
the mite-suger mix in water or alcohol to wash off the powder.

The trick is estimating the number of bees in your jar.