I doubt that strain guages or other "stress monitoring" techniques could identify fatigue cracks. One nasty fatigue crack running 12 inches in length had been found of what was left of the center wing box. This crack was masked by a doubler and would not have been visible during ordinary "stress inspections."
I'm sure that future Airworthiness Directives will mandate the removal of doublers to check for wing box structural integrity and corrosion more frequently. The problem is that it's difficult to quantify inspection intervals; how to correlate "fatigue cycles" into flight hours, given the extreme performance environment of fire fighting operations.