<A HREF="http://www.chara.gsu.edu/sat.html">The Earth Satellite Ephemeris Service</A> is a WWW-based service that will give you predictions for any given time and place. It makes magnitude estimates, and prints them in its output. You can ask it for only relatively bright objects. The magnitude estimates do not take into accout an object's detailed shape and reflectivity variations, so the actual brightness at any given time from any given point of view can vary significantly from the estimate.