Iodine starch test
An iodine starch test is useful to tell if all your starch has been converted to sugar during the mashing process. Take a small sample of liquid from your mash (a few drops is fine) and place it on a plate or in a glass. Make sure that it is free of solids. Add a few drops of iodine. If the liquid turns purple, there is still starch that needs to be converted. If the liquid stays the color of the iodine (reddish), then all your starch has been converted to sugar. See Uncle Jesse's Cooked rye mash page for some pictures of how this works.