How can one test the stability (sturdiness) of a Tripod?
It would also be handy if it can be done in-shop
When I want to test the sturdiness of a tripod I put it at its hihgest position and push down on the part where the camera would go. With cheap tripods you can see the legs move away from eachother (or even bend).
While pushing you can also move your hand back and forth a bit to feel if there's movement somewhere in the couplings.
You will also notice that there's less movement when you push harder downward. That's of course the same result as when using a sandbag (or so) for weighing down a tripod to give it more stability.
How about tapping the setup and immediately taking a time exposure? You can magnify the view in your camera's viewfinder. Not precise, but at least it gives a relative indication.