When or where are you talking about this purchase? If you are talking about the foreclosure auction, then the minimum bid will be posted with the sale notice, or you can get that info from the Committee for Sale.
If you are talking about after the bank has taken the property back, then there is no way to know except by making a written offer. They can then accept, decline, or counter offer.
Before making an offer, have an agent check comps to see what the house is worth. If you can justify your offer with facts, you have a lot better chance of getting it accepted.