Instructions
Check the bottle size. Before 1992, most commemoratives held either 10, 7 or 6.5 ounces. Since 1992, the figure is usually 8 oz.
Examine the bottle's shape. Most of the 10 oz. bottles have straight sides, although there are a few in the "hobbleskirt" mold. The later 8-oz. bottles are largely in the familiar hobbleskirt style, although there are also novelties such as the 2010 American Idol 9th Season "bulb" bottles.
Look at the labels. There should be four in all, two to each side, one of which will be a standard red label, while the rest will be commemorative in tenor.
Consider the color of the bottle. Some recent commemoratives have been made from aluminum, and you can also find 10-oz. bottles that have been painted silver.