What do you do when there is an cover on the undercarriage?
All suspension components are still visible and mainly accessible with lever bars, and bear in mind when the car is jacked up and the suspension is "hanging" (and lock to lock) there's access to pretty much everything, plus I also use a bigger version of a dentists mirror to check for broken coil/leaf springs etc where visibility is restricted.
And if I can't get a lever in there to check play I use the
shaker plates to check play in bushings, ball joints etc.
There really isn't much you can't physically test.
I think it very much is visual, i.e. only test what is visible.
My point, or at least the point I was trying to make was against this..
"There is no requirement for a garage to start pushing and pulling suspension bushes or driveshafts unless the problem is easily visible"
To be fair I'm not quite sure what the poster meant by "problem" as it's a safety check not a diagnosis session to find knocking noises etc.
A suspension arm bush can look perfectly ok, but get a bar in there and you can find that the bonding is defective and there's a massive amount of play in it and it's a failure.
Because something looks ok doesn't mean one doesn't test it.
But yes, in general, if something is covered up ie a sill has a plastic guard on it, then it's hard to check for corrosion or whatnot.