Social Security numbers are usually found in our patient demographic section, along with address, phone, religion, etc. If they are not in the system, I sometimes will access the SS death index http://search.ancestry.com/search/db.aspx?dbid=3693&cj=1&netid=cj&o_xid=0000584978&o_lid=0000584978&o_sch=Affiliate+External
and I look for the patient to have died in our county in Oregon. It probably wouldn't be as easy to do in a large metropolitan area, but people tend to not move around as much in our area. But I never delete based on someone with the same name who has died in Florida, for example.