...or, if you want the glass texture to look like actual glass (instead of just being completely transparent), you can texture that surface with most glass texture. This will allow any player to see through one side, but not the other.
My deathmatch map has an example of this.
http://twhl.co.za/mapvault_map.php?id=2557Go inside the courthouse and go to the judge's chamber. There is a one-way glass on the wall looking into the court room.