That's awesome!! My husband is a farmer and occasionally finds items in old house-spots that they're working up for the first time. Recently, he's brought home a handful of clay marbles, a petrified button, some arrowheads and a medium-sized medicine bottle.
My advice would be to locate the landowner first. The person on the tractor may not be the one who's name is on the deed, so ask about that immediately. Most farmers in this area don't mind if you hit them up early - before bedding and floating and planting and such.
Word of the wise though: in my experience, I've found that if they're zero-grading the grid you'll be turned down immediately and even if you *do* look, you'll find next to nothing after they're done.