there is a lot of things you can do to check the airframe for integrity. you can strip it back to its original off the assembly line configuration and weigh it to see how badly corrosion has eroded the body....you can do eddy current testing for cracks
None destructive testing has moved on since WW2 with huge strides. Eddy current, Dye penetrant ultrasonic (Lamb Rayleigh and other surface angle and compression wave techniques), but the basis of what you say is true. Warbirds were never designed to perform for 70 years just a few hundred hours.