In the olden days (well, 15 years ago) books were still the main way to check quiz questions. Even 5-10 years ago, the volume and quality of content available online wasn’t the same as it is now. Almost every famous person, film, band, TV show, organisation etc. now has an official site of some sort for a very reliable way of checking facts – but even then you find factual errors on official sites.
In the last few years, the biggest development for quiz writers has been the evolution of Wikipedia and its accuracy and reliable use of good citations. It is the fastest, most varied, most detailed and most well organised resource for quiz writers in the history of quiz question writing and to spurn Wikipedia is a very bold decision by any quiz writer (or at least a quiz writer who wants to work efficiently). Of course there are mistakes in Wikipedia, but there are also plenty of mistakes in other, supposedly/traditionally more reputable sources (both online and offline).
However, there is lots of nonsense on Wikipedia, and it does take a fair bit of experience/underlying knowledge/common sense to be able to spot the degrees of nonsense you might encounter. For example, with Wikipedia you always need to check the citations and external references, but no more so than with another source. The big difference is that Wikipedia is well enough organised, such that you almost always have citations and external references.
Oh, and of course Wikipedia is free. Anyone who writes pub and trivia quiz questions would, in my opinion, be a fool to ignore Wikipedia. But you would also be a fool not to check everything in at least two sources, regardless of what your starting point is for the fact.
Do you use Wikipedia as a tool for quiz question writing? Is anything else as good?