There are a few ways to tell if you have a really old antique tag.
The letters are not complete. No two signs are exactly the same. If it was perfect it was made using a computer and if more than one sign was exactly the same it was not written by hand.
– On the side of the lettered marks you can see the brush strokes. They have a slightly raised and uneven feel due to the coating.
– Standard size. Sign makers were practical using sheets of 4ft x 8ft, 4ft x 4ft, 2ft x 4ft, 18in x 24in, 12in x 9in, and so on. 28″ x 44″, so most display cards will be cut to that, in sizes like 22″ x 28″, 14″ x 28″, 11″ x 14″. Paper sizes of 8 1/2″ x 11″ were not standard at the time.
They’ve aged naturally. Really old marks don’t fade evenly.
– The materials used were common in that time period. The old “metal” signs were steel. Steel will rust. The magnet will stick to them. Newer “metallic” signs are usually enameled cooked aluminum. It does not rust and does not have a magnetic charge.
Ask yourself if that makes sense. The ceramic mark probably wouldn’t be quoted as these were very expensive and prices change.
It wouldn’t make sense if the edges were rusty and the rest of the sign looked like new. A rusted porcelain mark will appear anywhere where you have chipped porcelain and exposed steel.