There are a few tell-tale signs (without pulling back the husk) that a cob of corn with the husk on is no longer fresh. Light for its size. Fresh corn should feel heavy for its size. If it feels light, it means it's dry. Brown or black tassels. If the silks of the corn are dried out or black, the corn isn't very fresh. Missing kernels or mushy spots. Feel the corn through the husk from silk to stem. If it feels soft or like there are bare spots, pass on it. Once it's home, refrigerate and eat it within a few days. You can tell that corn has gone bad if it is slimy or has an offputting odor, mold, dark areas on the kernels, or the kernels are shriveled. If you are at all in doubt, toss it.