Left 4 Dead 2
"C:\Program Files\Steam\steamapps\common\left 4 dead 2\left4dead2"
Or similar path (C: may be different for example)
In here, open the "steam.inf" file with any text editor (like notepad) and see what it says next to "PatchVersion". As of today, it was 22.214.171.124 for example).