The closest you could get to original witchcraft would be to study the beliefs of early man, as well as the magick performed by twelfth- and thirteenth-century farming and working class European peoples.
A picture of the symbol you saw would do wonders, as far interpreting it. I know a lot about symbols and the like, so maybe I can help you there. If it's a sigil, though, they're unique to their purpose. Nobody can help you there, I'm afraid.
I can't help you with the third question. In my opinion, power is measured by each individual person; we each have a different criteria upon which we base it.
Well I can tell you that it isn't a sigil. Sigils are very specific, and generally don't use shapes like stars. I'm actually inclined to agree with you; it does appear to be some sort of modified triple goddess symbol, with the stars perhaps representing the elements and the cohesive moon symbol representing spirit.