There isn't a public source of information where you can find out how much they still owe on the property. Which is good -- I'm pretty happy that type of financial information isn't public record. However, if you go to the county recorder's office and look up the property, you might be able to make a pretty good guess. The mortgage lien will be filed, and depending on the county, might have some information about the original loan amount and interest terms. If you can get that information, you can extrapolate to know how much would be left on the loan if the payments were current. Of course, you won't know when the payments started to fall behind. The county recorder's website might have some helpful information. Some counties put a lot more online than others.
You might get a decent ballpark with more guesswork. You can find the owner's original purchase price and purchase date. Google historic mortgage rates for that date. Run the numbers with 20% down, and again with $0 down and a higher rate. You'd still be guessing about when the payments stopped.
Of course, you could ask the owner. I agree with the advice to be careful in approaching the owner, and that even if the owner is willing to sell, you will likely be dealing with the bank more than the owner. It would be a good sign if the current owner has been in the property for a long period of time, maybe 10+ years. Just because the owner can't make payments now doesn't mean the owner doesn't have positive equity. If the owner needs to get out of the situation, the bank won't care about the sale as long as the purchase price is more than the loan.