You can only see em if they are illuminated by sunlight. In fact, check out this SOLID site !
http://spaceflight.nasa.gov/realdata/sightings/SSapplications/Post/SightingData/sighting_index.htmlyou can find times when the space shuttle and the space station (ISS) fly over your area !!!! This is REALLY cool, especially when the shuttle is just about to catch or after leaving the ISS. Have seen em fly over many a time, is rad when they disappear right overhead because they go behind earth's shadow.
BTW - according to the original plans, when fully built, supposedly we would be able to see the ISS during daylight - but think the size was scaled back due to $