The adverse effects of ‘gees,’ as some pilots would call it, were known long before the early jet came into existence. They actually became known during World War One, when early dogfights forced pilots to conduct maneuvers resulting in such forces. However, in that period it was not well understood—it was simply an observed phenomenon. The number of accidents caused by G-LOC (which stands for ‘loss of consciousness’) in the 1910s is unknown and to the best of my knowledge nobody has ever attempted to calculate it as there wouldn’t be sufficient reliable data.

Research in this field was pioneered by Flt Lt (Dr) W.K. Stewart of the RAF Physiology Laboratory in the late 1930s. However, as this document (page 4) asserts:

the UK Flying Personnel Research Committee dismissed the “problem” by declaring that: (a) such episodes were "entirely typical of many aircrew"; and (b) unsuccessful pilots (certainly including those who suffered G-LOC) had already been shot down

As a result, the current level of our knowledge regarding gees stems from research conducted well after World War Two. But that is not to say that pilots were unaware of what was happening entirely. They very much aware and through trial and error a method of countering gees spread around the world. It involved flexing one’s abdominal muscles and elevating one’s legs (to the extent that space in the cockpit permitted) so as to restrict blood flow away from the brain. This was a crucial, yet underappreciated, trait of a skilled fighter pilot. Hans-Joachim Marseille, Germany’s foremost fighter ace in Africa, was famous among colleagues for his ‘physical exercise regimen of pushups and sit-ups, tightening his abdominal muscles to assist in high-G turns.’ (Heaton & Lewis, The Star of Africa, Zenith Press 2012, p. 50)

The anti-g suit became a thing only in 1941, which was early enough for early jet pilots (at least on the allied side, as that’s where it had been invented), and even early enough for pilots of modern piston fighters. The first combat use of the anti-g suit was in November 1942 (Operation Torch). However, researchers were still flying—so to speak—by the seat of their pants and it took years for the anti-g suit to be perfected through calculations and, indeed, trial and error. For this reason abominations such as the ‘Prone Pilot’ Gloster Meteor (see here, last photo) were developed. They were intended to explore other methods of countering gees, other than the still unreliable suits. This particular aircraft was first flown in 1953, which goes to show you that during World War Two the anti-g suit was still far from perfect.

The answer to your second question is, unfortunately, impossible. In the mid-to-late-1940s there were no black boxes to register the flight parameters. If a pilot crashed and died, he or she crashed and died and you could never be completely sure whether it was due to gees, a technical malfunction, or pilot error.