I think the National Republican Party has a platform (definition) of what the party is, but the GOP website isn’t loading right for me so I can’t find it.
The GOP in various states will also issue a State GOP platform. Here’s the one for Louisiana for 2008.
These would be the starting point for a “definition” of what it means to be a member of the GOP I would think.
Given Scozzafava’s support of card check and unions alone, I would have ranked her as a Democrat.