We have this problem with Standard Ebooks. The number of people that say things are public domain without actually checking it is very high. A CC0 licence is an explicit grant of public domain status by the licensor, and hence the legal issues rest with them in the event of any problem.

Public domain obviously can be ascertained, but if CC0 hasn’t been granted we rely on dated reproductions: basically a photograph of the artwork in question in a book or journal with a copyright date of 1924 or earlier.

(that’s obviously specifically a US legal reading, but SE from a legal point of view is a US project)