Hi Melissa,
The corn silk coming out of the end of the cob will turn a little brown.
Test one of your ears and peel back the husk and check it out.
Pick the corn as it's needed - the fresher off the stalk - the better.
For onions - you didn't say what kind of onions, but for the large
onions, you can somewhat see how large they are. I always used
a shovel to loosen the dirt first - made it easier to lift out.
Test one or two. If it's too small - eat the one you pulled, but give
the others a few more weeks. Onions can stay around in the ground
a while without harm.
When you do pull the onions - lay them out on the dirt where they grew
and let them alone on the ground for a day or two. This lets them season
a bit, and taste better.
Hope this helps. LeViolinist