Don't the bars start feeling "soft" or more flexy before they break? My take is if anything feels different about the bars, or the frame also, stop right away and get a closer look at it.

I check mine over every couple of years when I change the tape.

Mine didn't flex, it just snapped. Fortunately I was just accelerating from a stop, so I didn't even fall.

I did some posthumous metallurgy on it, most of the snap was clean/shiny but there was a gray part at the bottom, right where the stem clamp was. So I guess that corrosion/crack should have been visible if I had been looking for it.

That's the thing with fatigue; if inspection doesn't catch it, the final failure is so abrupt it's unlikely to be caught before something bad happens or almost happens.
In aerospace design, this is mitigated to some extent with "fail-safe" design, or at least "safe life", i.e. scheduled replacement of components with known finite service life.

Here's another photo from the bars in post #19, above. They didn't 'flex' when they gave, either. Just sagged down suddenly, and because I had so little weight on that side of the bar, didn't snap off all the way until I rode a bit further, bouncing down the gravel road with one hand on the left bar, and the other on the stem.

These are labeled "Belleri Made in France St Etienne" and are single-thickness, no sleeve. They seem pretty light. Almost certainly stock, as the other Trek I had of that vintage had the same bars. The little dark grey area is where the failure started, and so looks like the crack had been there long enough to oxidize a bit.



Almost home:

My 1974 Raleigh upright steel bar also just gave way where there was a rusting hole in the chrome.
It was on the right side of the stem. Got stuck with a 2 mile walk home. This bar was crashed several times making the right side pushed in. I did lately put it in the vice to fix it some.

So anyway it wasn't laid up long. I went to the co-op and found another perfect bar like new with a couple scratches. I used the CCM grips. It is an inch narrower, so maybe it will be faster. ha And besides that, I got to use a Nitto stem I bought 4 years ago.
Not sure I like using the aluminum though. I grocery bag it a lot of the time.