The shape of the tuber is rarely used ... and so not much help if you are just looking at it in a market and trying to figure out who's who. (some other tuber things are used, like skin color, inside color, etc.) The best taro key I have seen is from Bishop Museum in Honolulu. It is mostly about the Hawaiian taro varieties. The link is below. Once you get there, click "start" in the bottom right. THen take a break - it takes a few minutes to load up, there is a lot of info, pix, etc.
Hawaiian kalo