Reviewing upstream packages to write debian/copyright files is tedious but important manual work. It is done during initial packaging and after every new upstream release.
Making initial copyright file construction and subsequent review/update easier will improve Debian's software quality.
Starting with Stretch (Debian 9) there are significantly improved tools over previous releases to help.
Note that some of the tools listed here are run by check-all-the-things -f copyright.
scan-copyrights from libconfig-model-dpkg-perl can update an existing copyright file from rescanning the source. It can also create one from scratch. It uses licensecheck.
cme update dpkg-copyright
A script from cdbs can generate a copyright file using licensecheck:
licensecheck --copyright -r `find * -type f` | \ /usr/lib/cdbs/licensecheck2dep5 > debian/copyright.auto
license-reconcile compares the existing copyright with the source code and reports discrepancies.
debmake -k also compares the existing copyright with the source code and reports discrepancies.
debmake -cc generates a new copyright file from the source code.
decopy generates debian/copyright files.
licensee from ruby-licensee checks LICENSE files and returns known license names. This is the tool used by Githbub to provide a summary license indication on a repository main page. Its approach is to search for typical LICENSE file names or some package manifest (NPM, Bower, Gemfile, etc) and perform an exact or approximate license text matching against the set of common licenses texts as published at https://choosealicense.com (small: ~20). It output results in YAML format. This is a command line tool written in Ruby.
FOSSology is a open source license compliance software system and toolkit that can (in version 3.1) generate DEP5 copyright files. Its approach is to detect licenses with a either large (large:~6000 regexes) dataset of regex patterns (nomos) or a full string comparison against license full texts (large: ~400 text) (monk). It also detects copyright statements and can integrate with Ninka (see below). This is a complete database-backed web application with some command line support written in C/C++ with a PHP frontend.
LicenseFinder is a tool that "Find licenses for your project's dependencies." It does so by running application-specific package management tools and detecting package manifests to collect license-related metadata (e.g. Gemfile, etc) and detect licensing using regex against a set of common license texts (small: ~20). It output results in CSV, HTML and other report format. This is a command line tool written in Ruby.
Ninka is a "license identification tool for Source Code". Its approach is to detect licenses from text sentences using a dataset of key license sentences (large: ~600) and assemble the results based on the matched sentences. It output results in CSV format. This is a command line tool written in Perl.
ScanCode is a tool "to scan code and detect licenses, copyrights and more". Its approach is to detect licenses using a dataset of plain license texts (large:~1000 texts) and plain text notices (large:~2500 notices and mentions) and finds exact and approximate matches in source and binaries using full text alignments. It also detects copyright statements and collect license metadata from package manifests (e.g Maven, Pypi, etc.). It output results in JSON, HTML or SPDX format. This is a command line tool written in Python.
Updating debian copyright file with cme by Dominique Dumont
Creating, updating and checking debian/copyright semi-automatically by Petter Reinholdtsen