Copyright issues

I am sure any store that sells artistic content has encountered this problem. Music stores had their history with DRM, now they are trying to get rid of it. SnoCap is compiling an enormous database of all music ever produced. U-Tube is just saying that they are not responsible. So how do we make sure that artists publish their own original work and do not steal from other artists and their employers? For instance, one of our artists "3dd max" publishes very high quality 3D models. Recently, he uploaded a bunch of new models that look too good to be done so quickly. But can we be sure these are not his work? Of course, there are ways to check the history of the project file, but do we have to go to the length of getting the original project file form the artist and doubting his honesty? And furthermore, it really is not even feasible to check every single item for originality. Of course, when another artist writes to me saying that these were his models, I get the project files from both artists and punish the thief, but until someone claims their work, what can we do?