When connecting a ceiling fan in an old home and the existing light is wired black to white and white to black what should you do?

At that point, you should verify the home wiring; make certain that the black wire IS, in fact, the "hot" wire and that the white wire IS in fact the neutral. If the house is wired properly, connect the new fixture with "black to black and white to white."

If you aren't certain that the house wiring was done properly, contact a local electrician to perform the work for you. Connecting a light fixture improperly can be dangerous to you and to anyone who subsequently changes a bulb or otherwise comes in contact with the fixture.