I find that after the silk has been dry for about a week to week and a half they are ready. I just feel the husks. If the husks feel "loose" on the kernels underneath, it is not ready. If the husks feel tight against the kernels, it is done. Not real scientific, but it works for me.

marcantonio

vtgreenthumb has given you good advice. The reason you need to wait after the silks have turned brown is for the pollinated kernels to fill out and mature. You can also gently feel the cob through the hustks and should be able to tell if the top has filled out. It should be ready then.

One more thing you can do is gently pull the husk down to expose the top kernels. If the kernels are filled out, break through a kernel with your nail. If the liquid is milky, it's ready.