The runway choice is typically based on the wind. Headwinds are preferred. You can view the weather data for Seattle here: http://flightaware.com/resources/airport/KSEA/weather which will give you observations and a forecast.

If you look here: http://www.airnav.com/airport/KSEA you can find a phone number for the ATIS (Automatic Terminal Information Service) in the communications section which should give you the runways in use, but it typically changes each hour.