There is a MIL-SPEC tool used for structural assessment by tapping lightly on the structure. Not much better if at all than a coin, but it impresses the FAA when referenced in a repair procedure. I’m not making this up 
A friend took the tool to an FAA meeting anticipating the worst and when as expected the neophyte ex-military-NCO FAA cube dwellers said you can’t just do a coin tap test to assess a composite structure, he pulled out the tool and said we’ll use this MIL-SPEC tool instead, in compliance with DER generated inspection data (meaning a procedure that he had written and supplied to the DER). That was much better
Can I ask you why sad? There are a lot of things where a rough/ fairly good idea of its structural integrity can be heard by just tapping.
Most wood and fabric aircraft being a prime example. The sound the wing on a Jodel makes when you drop a coin on it, gives you a fairly good idea as to the state of it.
Not very scientific I know but not sad surely.😏
Oh the joys of plane ownership, 6 months into owning an almost 50 year old plane and a new AD comes out. I’ll do mine with my next annual.
It is funny/sad/curious that the determination of the structural integrity of a plane rests on the sound it makes when tapped with a coin.
The state of maintenance of said aircraft left something to be desired…
Scary indeed …
