This is a living page – more tricks will be added as I find them.
A lot of content on archive.org is in the public domain or is out of copyright. We can search for it using filters such as
rights:"Public Domain" or
But I’m still not convinced. The copyright of a scan presumably resides with whoever did the scan, but how can we find what copyright / license terms are applied by the scanner to a work uploaded to the Internet Archive? Does it rely on the person who uploaded the scan to ensure the license terms are embedded within the work, e.g. as many scans from Google Book Search digitisation project have a Google copyright notice at the start of the work (and watermarks on the scanned pages).