Info-index-nodes is a compiled Lisp function in `info.el'.

(Info-index-nodes &optional FILE)

Return a list of names of all index nodes in Info FILE.
If FILE is omitted, it defaults to the current Info file.
First look in a list of cached index node names. Then scan Info
file and its subfiles for nodes with the index cookie. Then try
to find index nodes starting from the first node in the top level
menu whose name contains the word "Index", plus any immediately
following nodes whose names also contain the word "Index".