Here is some info I got of the web!:Q: How do you know when an ear of corn is ripe and ready to be picked?
A: You know that sweet corn is almost ready for the cooking pot when the ears look plump and the exposed part of the silk, at the top of each ear, is dry and dark brown. To determine the exact ripeness of an ear, make a vertical slit in the husk near the top of an ear and use your thumbnail to puncture a kernel. The corn is at peak sweetness when the liquid in the kernels is neither watery nor opaque, but translucent.
Don't be fooled by poorly filled-out ears. If the silks are brown and the ear looks plump, but the kernels near the top of the ear aren't filled out, check a little lower where plump kernels may be maturing. When these are ready, harvest, even if there are undeveloped ones near the top.
Kernels that fail to develop didn't get fertilized. This can happen if plants are grown too close together or if there are too few plants in a group so the wind doesn't carry enough pollen to reach each of the silks, or if the corn is growing in a microclimate that is too chilly for proper pollen development.
To improve your chances of getting corn, be sure that the final distance between plants is 10 to 12 inches and that they are in square or round plantings that are at least 4 feet on a side.
Expect poor results with corn in the foggiest parts of the Bay Area, but in moderately foggy to sunny parts (in warmer parts of San Francisco and inland) you have until mid-July to plant a fast second crop. 'Early Sunglow,' rated at 55 days, may take as long as 90 days in moderately foggy areas, but still, that means you can harvest it just before the first October rains
Also don't feel stupid, I don't know when to pick corn either! (I only read the first paragraph!)