All the materials currently loaded in the scene are listed in the Materials List, including those that are not assigned to any object.
The Materials List panel
If a material has already been previewed in the material editor, a smaller version of the preview thumbnail will appear next to it. If the material has never been previewed, an icon with a N/A text will appear next to it.
...
Other than using the Materials List to list all the materials in the scene, it can be used to assign materials to objects. Just drag the material name over the object in the Objects List, over the object itself in the viewport, or over a selected triangle group in the viewport.
The Materials List right-click contextual menu
...
You can even assign materials to your objects automatically using the Assign by Name option, on the right-click menu. Use the combos to choose the material(s) to be assigned, and type in the blank fields the name of the object(s) that will receive each material. You can create your own rules like the one in the following example:
Assigning automatically materials using the Assign by Name panel
...