This is a quick tutorial for creating a one-way glass wall. It's an intermediate tutorial because I am going to assume you know the basics of mapping.
I have created a test map that you can download for reference.
Create your wall and texture it with whatever solid texture you wish to use. In my test map, I used a simple wall texture from the base_wall texture set.
Select the side of the wall you wish to see through using CTRL + SHIFT + LMB
Texture it with a glass texture such as glass2_nvp
If you don't want bullet marks to show up on the wall, draw a brush in front of the wall face you want to keep clean. In my test map, I drew a brush over half of the entire wall so that the entire covered half would not be marked by bullets but the other half would be.