5D2 is ok in the rain. If it is raining, don't worry to take a shot.
The 5d2, just like any non-1d camera is not really sealed - though the 7d and 5d3 might be a little better. Every shot in the rain thus is a gamble, rule of thumb is to avoid water *hitting* the camera as this will make the water go through the seals quicker. Better be safe than sorry, limit the exposure time, dry afterwards if possible in dry rice & if in doubt simply use a plastic bag.