There are a variety of codes and standards that your existing fire doors may need to meet or exceed, depending on the purpose of the building and its location. Existing fire doors can be certified to national or international standards, state codes and local building codes.
The 2007 Edition of NFPA 80, in addition to NFPA 101, provide a thorough guideline for the inspection and maintenance of fire doors in buildings such as hospitals. NFPA 80 states that fire doors must be inspected and tested at least once a year. Inspections must be recorded and signed. NFPA 80 also says that inspections must be performed by qualified individuals with an understanding of fire door technology.
In many states, licensed home inspectors inspect existing fire doors. However, not all states follow the same standards. A link to the International Code Council is provided in the Resources section of this article, which allows you to see which codes have been adopted in your state.