Feel through the husk: kernels should be plump and tight to the tip, and the silk tips should be brown and slightly damp, which is normal on a mature ear. Walk away from silk that is black, brittle, and bone dry, or wet and mushy. Older corn varieties turned sugar to starch fast after picking; modern supersweet types hold their sugar for days, but fresher is still better.
What to check
Lookbright green, tight husk; silk tips brown and slightly damp.
Feelrun a thumb along the husk, gaps or soft spots mean missing or shriveled kernels.
Skip the peel-it-back-in-the-store move; it dries the ear out for the next person and stores hate it. Feeling through the husk tells you plenty.
Written by the team behind RIPE, a free app that reads a photo of the whole produce pile and rings the ripest ones. Every cue above is one you can check with your own hands, and it all works with no app at all.