Based partly on comments to this question, how can a person who finds an image on a site, such as flickr, that hosts user-uploaded images verify that the it is in fact free to use. Suppose that the image's page on the site states that it has been released under a free license, such as CC_BY 2.0. But it is a known fact that some people have uploaded images to such sites, and marked them as being under free licenses, when the uploader did not have any right to do so. How can a would-be reuser avoid copyright problems?
Well, there is no simple and guaranteed way to do so. But there are some methods that can help avoid fake licenses.
One can check if there is a photographer's name in the meta-data of the photo itself. If there is, does it match the handle of the uploaded?
One can do a Google Image Search (or other similar search) to try find if the image has been posted elsewhere, and check if the stated origin / creator is the same. In particular does it appear on the photographer's site? Does it appear on a stock-photo site?
One can look to see if the uploader has uploaded other images, and if so how many. Do they look plausibly like the work of a single creator? Does the uploader have any online reputation, positive or negative?
One can reach out to the uploader. But a person who would supply a fake license tag, would, I suppose, also lie in an email.