There are lots of informed sources out there, including:
- The FSF, which has many articles on licensing and copyright informed by legal counsel and decades of experience. (e.g. I cited them earlier on whether small code snippets are copyrightable), and they have many other articles on what constitutes a “derived work” under US copyright law, since that’s necessary to understand the application of the GPL.)
- The Software Freedom Law Center, which has a useful primer
- People like Bruce Perens who have worked with legal counsel on free/open-source licensing for many years have written many useful articles.
- Debian-Legal has a useful FAQ.
- Lawrence Rosen, a well-known lawyer specializing in software law, has a book on open-source licensing and a useful article on derived works, although the FSF disagrees with his assessment of whether linking creates derivative works.
- …
As you pointed out above, perhaps the least reliable method is to argue from the principle of what you think the law should be, unfortunately.