Fryovanni,
I believe we may be talking about the same thing. Correct me if I am wrong...
Only the geometry of the instrument will determine the vibration pattern. Assuming all other properties (density, etc) are constant throuhout the instrument, these will only affect amplitude. This being said, a node is a point on that pattern where the amplitude is zero. Holding the guitar as you do at a node, and tapping on a non-node location will yield a tone. As you tap closer to a point on that pattern where the amplitude is maximum, the louder the tone will be. This is a way to determine nodes and areas of high amplitude.
What I was doing was holding the guitar at a point far from where I want to test, (i.e. the headstock), and tapping around the body. Here you can find the nodes in slightly the same manner. The dead areas are the nodes. Granted, holding the guitar in any place will change the pattern, but the headstock should be far enough away for all practical purposes.
What I was curious about, was since my guitar looks similar to a fender, and I had noticeable nodes at the bridge area, is this something that is also unique to most solidbodies?...