The tassels will have just barely turned brown and the few kernels showing at the top of the ear will be whiteish. If you need to pull back a little of the husk... a fingernail applied to a kernel will produce a milky center. Pick earlyinthe morning for best results...
Try to cook the heads of sweetcorn within 20 minutes of picking,after this the sugar in the kernels turns to starch;still nice to eat but a world away from the pre 20 minute ones.