info-xref-subfile-p

info-xref-subfile-p is a compiled Lisp function in `info-xref.el'.

(info-xref-subfile-p FILENAME)

Return t if FILENAME is an info subfile.
If removing the last "-<NUM>" from the filename gives a file
which exists, then consider FILENAME a subfile. This is an
imperfect test, probably ought to open up the purported top file
and see what subfiles it says.