Most Library materials are listed in Stanford University Libraries' online catalog. You can limit your search to "Hoover Library" in the left sidebar under "Library." Some materials acquired by the Library before 1977 may be listed only in the card catalogs located in the Library.
The Online Archive of California (OAC) contains the most complete descriptions of all of the Archives' collections, including detailed collection guides (finding aids) when they exist. This is the richest information for searching. By entering keywords in the search box on this page and choosing "Archival Collections" you are searching all of the Archives' finding aids.
You can also go directly to the OAC and browse to a particular finding aid by clicking on the collection title in the alphabetical list, then clicking on either "PDF" or "HTML" under "View entire collection guide" in the shaded box to the right of the collection title.
All Archives collections also have a short description in Stanford University Libraries' online catalog. You can limit your search to "Hoover Archives" in the left sidebar under "Library." If an archival collection's catalog description includes the words "preliminary inventory," "inventory," or "register," there is a more detailed finding aid available at the Online Archive of California.