My question is how buyers at the sale barns know I have precondition my calves.
Buyers know when a calf has been truck-weaned because of the way the calf looks. A freshly weaned calf goes off feed. He will have hollows in front of his hip bones, he will also be more agitated than usual, and bawling.
Are they just guessing based on the behavior for a few seconds in the ring?
No, they are not just guessing. When you've been around cattle you can tell which calves have been truck weaned, and which ones have been fed out following weaning.
Are barns sales always considered unweaned.
No.
If I have my calves weaned for a couple months will the sale barn buyers know it?
Generally speaking, yes. Now, having said that, I think you also need to bear in mind that sometimes calves wean hard and it could take that long for them to really start eating and gaining weight. I would suggest you visit a few sales, really look at the calves, take notes on what weights are bringing good prices, and what those calves look like.
I don't see how they can especially for small lots. I have tried to sell groups of uniform calves so far, but what I have left now is a mixed bunch with different colors.