I totally agree that the specific example you linked should be tagged knowledge (in fact, I've edited it in). In this instance, it could even have been music, but that may be too much of a hint, and I don't mind people using slightly more ambiguous versions of tags to help hide information, just not complete omission. However, your more general question is a little more difficult to answer, beyond "I'll know it when I see it", but let's have a go at codifying...
You should use knowledge if you answer "yes" to any of the following questions:
1. Would the "average person" be oblivious to the subject/fact?
If I took a bunch of reasonably intelligent people at random, would less than half of them even know of the information required? So people like:
- your mum
- a teacher
- a work manager
- the news anchor
- publican from your local
- any of the above, but from a country on the other side of the world
Note that I say "know of the information", so they don't necessarily have to know the details, but they'd need to know what to google for to find the specific answers. I'd expect most/all of the above to recognise that The Beatles is a band (even if they didn't know any songs), maybe one or two to recognise Metallica, and none to know of Igorrr. Similarly, I'd assume most could name a few US presidents (even without being American), a couple to know a French president or two, but none to know any ex-presidents of the Maldives.
If a majority of average PSE visitors would not be aware of the information required, then you should use knowledge.
2. Do you need the appropriate knowledge before you start working on the puzzle?
The direction in which you work through the required knowledge in a puzzle is important. If a puzzle makes clear (or hints heavily at, or indicates after solving the first part, etc) that you need to know the names of the bones in the human body then the solver can google a list to assist them continue on without requiring them to have that knowledge up front. If the puzzle requires solvers to stumble across "coccyx" during solving and recognise it as the name of a bone, then that's less fair.
If a solver can't derive the required information based on the puzzle itself (and obvious subsequent googling), then you should use knowledge.
3. Is the required knowledge difficult to obtain quickly/easily?
If you've got past the first two questions, the third hurdle for the solver would be whether they could easily obtain the knowledge required once they know they need it. If all the information is available, say, in a single Wikipedia list page, then it's probably OK. If you need to reference multiple web pages or use tools to gain the knowledge then it's probably not. If you need a specialised degree (or access to the textbooks thereof), then it's certainly an issue.
If a solver can't find information they know they need in a single, obvious location, then you should use knowledge.
If your puzzle relies on some form of outside knowledge, but you can answer "no" to all three of the above then you probably don't need knowledge (or music/movies/etc). If any of them is a "yes", then I think it is probably warranted.
That being said, all of the above has been about trying to specify when knowledge should be a required tag. It would still be perfectly acceptable (and even preferable) to include it even if you answered no to the three questions when your puzzle requires having/applying specific knowledge. Personally, I'd still use it if any sort of knowledge beyond, say, a high school level were an integral part of the puzzle (which is probably true if you even need to answer question 3 at all).
