Open the end of one ear and take a look. If the silks are brown it should be ready. If you need to break one kernal witha fingernail, if it is milky it is at optimum stage. It can get too mature (hard) very quickly.

My corn is ready to harvest about 3 days after the racoons think it is ready

Choose one ear and open the very top so that you can see mature kernels. Using your thumbnail, cut into the kernel. The milk should spurt out over your thumbnail. That's ready. If the kernels are still small and sort of pointy, it's too young; if they're tough and don't give up milk, it's too old.

You can still cut old corn off the cob and freeze it, it'll just taste more like store bought canned corn.