You write an offer to the seller.
You will want to know if the seller and their representative have been in communications with the sellers lender and if the lender has told them what they are willing to accept for a sale price. If the homeowner and their representative have not yet contacted the lender about a short sale, I would move on to the next house.
Keep in mind that short sales can take longer to close (as compared to traditional sales) as the seller has to get approval from their bank as part of the sale process. The whole short sale process has improved, but it still takes time. And if the bank rejects your offer, you might end up back at square one.
So if you are in a position where you have to move by a specific date, or if you need to sell before buying a new home, then short sales might not work for you.