There are a couple of ways it can be done. It is public record so the information isn't private. Depending on the State, many have online access (free and for a fee) to public land records. Otherwise, go to the county courthouse and look it up manually. You can probably pay an online information service to do the work for you for a nominal fee.

Start by Googling up " _____ county public land records" and see what pops up.  

I think they were asking how to find out how much the seller still owes on the home.  I didn't think you could find that in the county records, only how much the original mortgage was for.