There are several ways to tell if you have a genuine antique vintage sign.
-The letters are not perfect. No two signs are exactly the same. If its perfect, it was done with a computer and If there are more than one sign exactly alike, they were not lettered by hand.
-On hand lettered signs you can see the brush strokes. They have a slightly raised and uneven feel because of the paint.
-They are a standard size. Sign makers were practical using 4 ft. X 8 ft.sheets, to 4 ft x 4 ft, 2 ft x 4 ft, 18 in x 24 in, 12 in x 9 in, etc. Full size show cards and poster boards are usually 28 "x 44", so most show card will be cut down from that, in sizes such as 22 "x 28", 14 "x 28", and 11 "x 14" sizes. Paper sizes 8 1/2 x 11 signs were not standard then.
-They have aged naturally. Genuinely old signs do not fade evenly.
The materials used were common to that time period. Old "metal" signs were steel. Steel will rust. A magnet will stick to them. Newer "metal" signs are usually a baked-on enamel aluminum. They do not rust, and have no magnetic charge.
Ask yourself if it makes sense. A porcelain sign probably would not have a price shown because those signs were expensive and prices change.
It would not make sense if the edges were rusty and the rest of the sign looked new. A porcelain sign will show rust in any spot where the porcelain has chipped off and the steel was exposed.