There are several ways to tell if you have a genuinely 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 weren't 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 don't 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.