I am not a lawyer, this is not legal advice.
You can't in general prove that something published post-1923 is public domain. We do not have (any more) a requirement to register copyrights, plus we now have very long copyright terms. Either would be bad, the combination is (quite deliberately) pernicious. It means that, while you may have good reasons to assume that something is PD, you can almost never know for sure. There are two major exceptions - works that have been declared to be PD by their owner, and works by the US Government (which are PD from birth). US Government works are generally pretty safe, but works declared PD are not always (as you have no way to prove the donor actually owns them).
If you think that this implies that our copyright laws need to be changed, you are IMO correct. I would go for a term of 14 years, one renewal possible, with registration required. The wailing from the rent-seeking entertainment industries would, of course, in that case be something to behold, but that would have some entertainment value in its own right.