How do i know what Steam has downloaded

[wedge22]

I am a new user to steam and I cannot find out what it has downloaded, where does it store files. I would like to know if it has completed all the phases of the current HL2 pre load.

[directhex]

right click on an app in the list, click "properties" for an installed game or "pre-load status" for a non-installed game, it'll give a completed percentage.

data is stored in large .gcf data files in steam/steamapps