If can find the Parish and Vital Records Index, this will list those films that were extracted for the IGI. This was on microfiche, and available in the various FHCs. The Centers have generally gotten rid of this, but a few still have it. Some parts have been posted online in various forms.
This index would give the catalog numbers of the extracted films, and was a significant aid to IGI searches. It still has a bit of utility, but not very much.
This has been rendered pretty much obsolete by the current FamilySearch catalog which tells you if a film is only film, or extracted/indexed and digitized. From the catalog is has become easy to search a specific film, if extracted, but just clicking on the magnifying glass icon.