how to tell what is in your level

[rex64]

How do I know which textures... are the games and wich ones were automatically downloaded from multiplayer levels that I have played in the past? Same thing, how do I know wich objects... are stock.

 

[WAR_Nuker]

any texture you used that was available in the browse menu is strictly from valve. there will be no textures in there that you downloaded from other maps, unless you manually put those custom textures in there.

 

[Raeven0]

Any materials in the materials folder will be displayed in Hammer, including but not limited to downloaded materials.

The easiest way I can think of would be to rename the materials folder, then open the map again. All custom materials would come up missing.