I believe one's credibility is self made, and one's reputation does not define truth but is instrumental in how the community decides to credit many works.
These days we have a number of independent services (TwistyPuzzles, flickr, etc.) that can widely be trusted as "truth" when it comes to the timing of activities. But even such services are only as useful as when people use them. So plenty of us were building long before any of these services existed to be used as a verification of our work, yet our reputation and the credibility we have built is accepted nonetheless.
Good examples are Tony Fisher's works, my Square-2 and Aleh's 4x4x4.
I think for most of us creating puzzles is is own reward, and while a race to be first may have some appeal it is not the goal. I have seen few if any dedicated builders find anything but pleasure to discover someone else made something similar (for example Jason Smith and Bob Hearn on the Face Turning Icosahedron).
So this site has no objective required measures for proof, but we do our best to assess the proper history and give credit where due. How any particular claim is assessed will logically be in the context of how the staff and community view its credibility.
Dave [Moderator] I copied this post from here. If you are interested you'll find a discussion there.[/Moderator]