The library's online catalog, also known as NYMCat, is an authoritative source for information about the library holdings for books and journals. Looking up a journal title, for example, will provide you with information about what years and volumes are available and whether the journal is available online or in print on the library shelves.
A link for the catalog (listed as NYMCat) is located on the library homepage under the Resources tab and in the More Research Tools box.