Given the quantity of porcelain repops and the number of people making them, I don't see how anyone can be CERTAIN that a specific sign is authentic. To say a piece is "original" assumes you know the manufacturing capabilities and talents of ALL the people making porcelain knock-offs and that none of them could possibly make a sign of the same dimensions and characteristics as the originals, and/or "age" them with that "antique look."
This is particularly true if you consider that many originals were manufactured over a period of time and during those years there was ample opportunity for sizing and screening techniques to vary slightly. In some cases, oil companies used different sign makers to create their advertising, thus creating yet another possibility for slight variations.
IMO, the only way to have porcelain sign authenticity assurance is to buy from someone you know and trust, who assures you the sign is original and was obtained at least twenty five years ago.
