If the field corn sheds pollen at the same time as your sweet corn forms silks on the ears, they could indeed cross pollinate and cause some starchy kernels. Most sweet corn varieties are quicker to mature than field corn, however, so if they were planted at the same time, your sweet corn may be done with pollination before the field corn begins shedding pollen. Separating field corn (or different classes of sweet corn) by time or distance is the only sure way to prevent cross pollination.