I remember seeing euro pro riders in the 1970s who would walk up to frame and perform a "test" on it. The test consisted of grabbing the stem in the left hand, the saddle in the right hand, and pressing one's shoe sole directly onto the left crank nearest to the BB when it was at 6 o'clock. They were, by feel of the flex in response to the laterally exerted force, diagnosing the "life left in the frame" before it "needed to be replaced as worn out". That being said, try to tell it to a friend of mine who is nearing 200,000 miles on a PX-10.
Ken, near St Louis