Determining Brightness using the Magnitude System
Thus at one extreme the Sun is magnitude (mag.) -27 and some of the faintest observed stars around mag. +24. The full moon is mag. -12.5, Sirius the brightest star in the night sky mag. -1.5, whilst the faintest stars visible to the naked eye under good conditions are around mag. +6.
The advantage of this method is of course that the stars are readily at hand for comparison with a satellite (given knowledge of stellar magnitudes).
Further discussion of the magnitude value for brightness can be found on the Bright Satellites & Resources page.
A quick guide to conditions and satellite brightness may be gleamed from examining a suitable constellation. In the Northern hemisphere Ursa Minor (the 'Little Bear') is ideal. Circumpolar and thus often visible, it contains stars covering magnitudes +2 down to +6. Brighter apparitions can be gauged by spotting some of the more brilliant stars (Sirius, Altair, Vega, Deneb, etc).
This diagram indicates the magnitudes of the component stars of Ursa Minor; an excellent guide to seeing conditions.
This diagram indicates the magnitudes of the component stars of Crux as a guide to seeing conditions.
Here again, acquaint yourself with a suitable starfield (naturally you will need a good star atlas this time) in order to determine the seeing conditions. By noting down the path of the satellite through your field of view, and detailing stars of similar brightness, you can refer back to the atlas once more in order to figure out the satellite's magnitude after the pass.
Of course following a satellite in a large telescope needs a motor driven mount, with accurate co-ordinates for the pass. Even when using valid, up-to-date USSPACECOM elements, the tracking error can amount to up to one degree. This is even without considering the maneuvering that the shuttle will perform regularly. Still, images can be obtained.