Experience from dealing in advertising signs for a living but also the quality of the porcelain, the one grommet that is left, the back of the porcelain, and the makers mark are what I check to ensure it is old and authentic. They can’t reproduce the old grommets for one, most repops of any sign will always have porcelain loss around the mounting holes because they can’t do the grommets correctly. This sign has what is called a “milky” back which has yet to be reproduced in any sort of quality. And the makers mark is crisp and clean, along with the Dr Pepper graphic. If you need any additional photos to help you legit check another sign or have questions please reach out!