Sometimes there are clues in the material itself. You may have a third edition, dated 1975. Statistical tables should indicate on the table the date of the information given. Tables usually give the name of the group or agency that compiled them. It's worth checking to see if later figures are available.
Office holders change very fast.
Directories are generally out of date before they are published. If it is, offer to pursue the question, giving the patron a realistic idea of when you might have the updated materials. If the patron can't wait for the referral process or prefers to make a direct contact, offer a source where the patron can go directly to get updated information. For example, check the statistical table to find out who compiled the data, and use a directory to get a current phone number. Or, if a person is looking for the name of a current office holder, offer to supply the phone number of the government office.