What is a tool for locating magazines in a library called?

There are several ways to find magazines in a library. My favorite old-school way is using the "Readers Guide to Periodical Literature," which lists by author and by subject the names of the articles in most major magazines from the late 1800s to the present. (It is now available online.) Most libraries also have an index which tells you which magazines they own; these days, that index too is generally online. There are also magazine databases like Academic Search Premier that most major libraries have.