How to tell when to change steel front fork? 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!
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).