I'll post a few comments to help the readers of this thread. I'm NOT the authority nor do I want to be, but this is what I have learned over the years. I started collecting about 40 years ago. I made an attempt to buy American Porcelain in Dallas (in about 2005) that had been in business since the late 30's. They made all of the New Jersey transit signs. I spend several days in Dallas talking to the employees/managers who had been in the business 30 years. I watched them make signs and asked how signs were made in the 40's vs. 90's.
This is what was taught to me at the factory:
Old signs (before computer graphics) were "hand cut stencils" with an exacto knife. This means if you look close and the sign you are looking at has 2 - "O"s, each "O" will be slightly different because it was hand cut. Computerized graphics will make both "O"s exactly the same. This test will catch a LOT of signs.
Look at curved/arched letters that go around the perimeter of a 42" sign. Are the letters spaced properly? When "fakes" are made, these perimeter letters are hand spaced (copied and dragged) in place. In most cases, the spacing is slightly off, the angle is slightly off on a couple of letters, and it looks like an ugly sweater. You can't quite telll what looks wrong, but something does.
Barometric pressure effect firing and boiling point of frit (the compound fired to make porcelain), therefore "color" which are not stock colors are very hard to match. It is not like mixing paint, because there is no paint in porcelain and the color doesn't reveal itself until after the sign is fired. This is why the green T in the Texaco logo is so hard to match. Or the blue in the Magnolia signs are so hard to match.
These porcelain sign companies who were contracted in the 40's from these huge oil companies didn't put out junk. Although the signs were "production" signs, they had quality control and high standards in place.
If you take all of the obvious reasons, comments listed above, and apply some of my knowledge, it will be hard to get burned. AND lastly, unless you know the seller and he will not guarantee your satisfaction DO NOT buy from a picture because you can't always examine everything.