Back in the "Good Old Days" the DC8-50, and maybe other DC8 models, had a small strip of Al clamped to the bottom of each MLG strut pointing up. Part of the engineers' turnaround check was to see if they were still straight along the strut. If bent, it was a heavy landing and an inspection required, if straight no problem.
We called them "lie detectors" and it saved any arguing the toss with the pilots.
