That was what I was wondering as well. OK, quick google says:
"Harvest sweetcorn as it ripens. Push a fingernail into the kernel when the tassels at the end of the cob start to shrivel and brown. If the liquid is milky it indicates they are ready." From the Royal Horticultural Society, so should be quite reliable.
Not sure when you should pick sweetcorn but it's worth cooking as soon as you possibly can after picking. The sooner you cook it, the better the flavour. I remember my husband running up the garden with freshly picked sweetcorn while I waited with the pan of boiling water. Sounds silly now but I swear it was the best sweetcorn I've EVER had I haven't grown sweetcorn for years (smaller garden now) and I'm rather jealous
Oh, now you see I have always just planted it, then waited for an unusually large green-skinned man wearing a small skirt to pick it for me. He then pops it into nice tins as well. I suggest you wait a while to see if he comes.