After their wings dry, that can take up to 2-3 hours depending on the weather, you can take a permanent marker and mark the underside. I put 'M's on the underside. I had females stay in the late summer in an area that had abundant milkweed and nectar sources for up to 5 days.http://www.monarchwatch.com/read/faq2.htm#4
"4. How fast do individual migrating Monarchs fly?
Clearly, the flight speed of such a small organism depends on environmental such as wind speed and direction relative to the heading (direction) of the butterfly. Monarchs are slow fliers relative to other butterflies and moths, flapping their wings only 5-12 times per second. Their flight speeds have been measured at 5 meters per second, 18 km or 12 miles per hour; but, if disturbed, they can fly much faster for short distances...."