That is a good question that I raised a while ago. For the lack of a better authority, I have decided to go along with the Cartographic Laboratory of the University of Pennsylvania who has created a very nifty web presence for neighborhoods called the Neighborhood Information System (NIS).
Contained in the NIS is a piece called the neighborhoodBase. Browse around there and have a look at what is available if you have not been there before. In their quest to provide useful information they have taken up the task of deciding where each neighborhood begins and ends. I don’t entirely agree with how they have arrived at Frankford but it is close enough for my purposes.
This is a link to the interactive map of Philadelphia that will let you display a map of each neighborhood in the city defined by Penn. Take a look and see if you agree with them.