From Unreal Wiki, The Unreal Engine Documentation Site
Texture Cull is an operation that UnrealEd can perform which will remove unused textures from your map and also give you a list of all the textures currently in use. To perform a Texture Cull, type
into the UnrealEd Console. Open the Log window (UnrealEd Main Menu -> View -> Log) to see progress. UnrealEd will give you a list of "referenced" textures (in use by your map), and "culled" textures (ones that aren't in use and have been removed).
Do NOT use this on a partially rebuilt map. Any textures which are only used by hidden brushes will be removed & the surfaces that used those textures will be set to the ugly green bubbly default texture. A 'hidden surface' in this context is one that is not visible in the map but theoretically should be there - it simply is not reflected in the game or the 3D view because the BSP geometry has not been rebuilt since the brush was moved. In particular, don't run this command on a partial build.