There are several tools to proceed to the review: decopy,
scan-copyrights, licensecheck. They may behave slightly
differently but mostly do the same thing. I personally mostly
use scan-copyrights to bootstrap my entries, but I can't really
tell you which tool is the best, you may have to pick a few ones
and give them a try. In any case, once filled with automated
information, I try to crunch the result to make it more easy for
humans to read, and manually check every files for copyright
information.