Before you start, download Crowbar. Open the page below and scroll to the download section.

Once downloaded, extract the archive into a folder.

Crowbar - Source Engine Modding Tool

1. Open Crowbar and head to the View tab.
2. Here, click Browse… next to the MDL file field and select your file.
3. Scroll down to Material File Names, the list contains all materials (.VMT files) used by the model.

Once you have the material (.VMT) file, you can open it with any text editor to check which textures (.VTFs) are needed for that material to work.