How do can I tell if I have already downloaded a material?

At this time GameTextures.com does not yet have a visual, at a glance indication on the main library page for an already downloaded material. 

This information can be checked what you have downloaded by looking in "My Library",

The other way to check requires selecting a material. 

If the material has never been downloaded before it will indicate a download cost:

If it is a previously downloaded product it will only indicate your ability to download it again: