The best way to find out the remaining mortgage balance is to ask. And ask about all mortgages. If there's a second mortgage on the property or a HELOC, that counts, too. Then, if you want to be sure, get an "authorization to disclose" signed by the seller and contact the lenders yourself.
However, as some noted below, the amount owed has little relationship to the offer you should make. It may have some effect (the first mortgage, in the case of multiple liens) on what the lender might accept. But there are plenty of houses out there that aren't working as short sales because the amount owed is way above what the house is currently worth. So (to cite a local example), if someone bought a house for $550,000, zero down, a couple of years ago, he still owes $550,000. But the house itself, in today's market, is only worth $350,000. Lenders vary, but relatively few are going to let that house be sold for $350,000. They're more often looking at something in the range of $450,000. Yes, it's a $100,000 "bath" for them, but you'd still be drastically overpaying.
Hope that helps.