so...if instead of tapping the guitar, you tapped a table - as hard as you want with whatever you want (except the guitar, a different guitar is OK) - and that came through the amp, you would have conclusive evidence of a microphonic tendency.
Yes, that would be a clearer indication, but what we know about pickups and microphonics is that the movement of the wire winds around the magnet are what creates the phenomenon. Potting is used to prevent this movement, to a greater or lesser degree, depending on the pickup.
In this way the wire winds around the pickup's magnet behave somewhat like a moving coil microphone's coils and magnet.
http://www.neumann.com/homestudio/en/what-is-a-dynamic-microphone
One has to remember that it isn't the diaphragm that causes the electrical signal in a dynamic mic, it's the movement of the coils
attached to the diaphragm, moving relative to the stationary magnet (in certain designs the magnet moves instead, but it's the movement of the wire relative to the magnet that creates the signal).
This also happens to a much smaller degree when the wire winds around a pickup magnet respond to vibrations -- even though they aren't designed to facilitate that movement, and even though they aren't attached to a traditional diaphragm. But to a degree, I'd say that the body of the guitar itself can behave like a diaphragm on a microphone.
Without a specially designed diaphragm to pick up the noise, it takes a more physical vibration to move the windings of a pickup relative to the magnet, compared to a mic. So a microphonic pickup is going to need a lot more vibration, which it gets from being attached to the guitar body. It's not going to be vibrated enough by air, except in the case of feedback.
While the guitar body becomes akin to something like the mic's diaphragm, its movements from vibration are much smaller. In fact, the reason a hollow body guitar is more sensitive to feedback is simply that the top of the guitar is thin, and freer to move; therefore, it is more likely to behave and vibrate like a diaphragm. When the top moves a lot, the wire around the magnets in the pickups moves a little. And you get feedback.
This is why the solid body electric guitar was invented.
A solid body guitar, on the other hand, is less likely to behave like a diaphragm because it isn't as free to move due to its thickness, and is therefore less likely to in turn vibrate and move the wires around the magnet in the pickup, and therefore, feed back.
But that doesn't mean it doesn't vibrate when a string is plucked, or when you knock on the guitar body.
Because they don't have a proper diaphragm, microphonic pickups aren't as sensitive to sound waves as a mic. So they're more likely to respond to vibrations in the guitar body than to picking up noise external to the guitar.
However, very loud signals from guitar amps do affect microphonic pickups and make them howl - and we've all experienced this as feedback, and the reason feedback can be musical is because it's reinforcing the note the player is playing via both the vibrating guitar and the vibrations from the loud amp.
In any case, I don't think you're going to get much knocking on a table unless the guitar is sitting on the table, because in the case of a microphonic pickup, to a degree the guitar body becomes the diaphragm of the "mic."
Please forgive me for not using scientific terms here, but I'm trying to explain what I know about this without the benefit of equations and so on and while I understand how this all works, I am not able to explain it as easily as a scientist or mathematician can with equations and the like.