I am riding a Dawes made sometime before 1974 and have read in a Swedish report (http://vti.diva-portal.org/smash/get/diva2:694821/FULLTEXT01.pdf) that front fork failure is a not uncommon cause of sometimes serious injury. However, I cannot find any tips on how to know when to change my fork? All tips and advice would be much appreciated!
My experience is: Any bike part can, and will fail under heavy use. This includes frames, forks, stems, seat-posts, handle bars, cranks, you name it. I have broken at least one of all these in my life already.
Obviously, the risk of breaking things depends on some variables:
The distance that you've ridden.
The street conditions and tire pressure
(how much/strong vibrations does your bike have to endure).
If your usage is particularly heavy for some reasons, it might be worth routinely replacing crucial parts like fork, stem, and handlebar. I have decided not to trust these parts for more than 10000km, simply because each of these has already failed me at least once.
The 10000km that I use are just a round number that fits with my experience of my failures, and it's heavily dependent on my bike usage, which is rather rough. Your usage is likely less stressful on your material, and may thus warrant a longer replacement interval. Also, the 10000km take into account, that a handle bar failure is always catastrophic. I do not want to experience any more of these failures. Thus, I try to err more on the safe side. YMMV.
Forks are not a 'wear component' that need to be replaced periodically. Bicycle frame makes are clever enough to make their products sufficiently durable and it's not something riders generally need to worry about. That said there obviously are instances of frames and forks cracking and failing, but this is not the norm.
However, in your particular case, with an old steel frame, it's a good idea to periodically inspect it for any cracks that might be developing in any of the welded or brazed joints. On the fork you should look for cracks near the fork crown as that is where forces are highest.
As as aside, I haven't tried to put the document through Google translate yet, but there may be a slight misunderstanding. I think perhaps you are interpreting it as: bicycle forks commonly fail, leading to injury, when perhaps that should be if a bicycle's fork fails, injury is common.
Personally, I've never seen or heard of a modern bicycle fork catastrophically failing in regular use (crashes and extreme mountain bike riding are another matter).