One way of telling if your corn is ready is by looking at the little tuft of fine silky corn hair that hangs out from the husk. If it is nice and dark till the tip of the husk, it is ready to be harvested. If it is still pale and light in color, then you ought to wait a little longer.
You can also peel back the layers at the very top just a little and see if the corn kernels there are ready. If they are, then the rest of the corn is ready too. You can also tell if they are nice and juicy by feeling them or pricking them. This is perhaps the best stage to harvest them. If you wait longer, as you say, it will taste starchy and chewy.