"From South Dakota" joke

A young fellow at the state fair stood watching an old man. Above the old man was a sign that read, "$5.00 - If I can't tell you where you're from, I'll pay you $50.00!"
The young man watched a cowboy approach the old man and ask, "Is the sign right?"
The man says, "Yes." The cowboy hands him a five and says, "You're on!"
The old man looks the cowboy up and down, noticing some cow dung on his boots and flatly states, "You're from Wyoming."
The cowboy shakes his head and says, "I'll be darned! You're right!" and strolls away.
A second cowboy approaches the old man and goes through the same routine.
Handing him the fiver, he stands and watches as the old man looks him up and down and notices a bit of straw and cow dung on his boots. The old man says, "You're from Montana!"
The cowboy, dejected, walks away.
The young man decides he's going to give the old man a run for the money. He goes into the mensroom, takes his boots off, scrubs them, dries them off and puts on a coat of polish. He walks up to the old man, hands over a five dollar bill and says, "Do your stuff!"
The old man looks and looks, up and down, and appears to be befuddled. The young man is now thinking he's gone one up on the old geezer. Finally, the old man says, "You're from South Dakota!"
The young man gets really upset and can't for the life of him figure out how the old guy could know that, so he asks, "How in the world did you know I'm from South Dakota?"
The old man replies, "By the wool on your zipper!"