How do you tell the difference between sweet cornand field corn?

Sweet corn is just as its name implies, it is sweet. Field corn is not sweet.

Also, sweet corn is usually for human consumption at an immature stage of growth as the soft kernels that are shaved off the cob, or to be sold as corn on the cob. Field corn is normally allowed to fully mature and dry down. It's usually used for ethanol production or livestock feed, and in grain-form for cereal products including breakfast cereals, harmony and grits, as alcohol and corn whiskey, and other human foodstuffs like starch, oils and sweetners.

From a genetic standpoint, the expression of the Su1 or Sh2 recessive genes is what makes corn either sweet, field, or some other type. Please see the related links below for information.