casting - How to correctly identify an actor/actresses is in something that is not listed on their IMDB profile?

IMDb credits are generated by the filmmakers themselves (for smaller, independent productions) or representatives for the filmmakers/producers.

On occasion crew and cast members get left off the submission form. I speak from experience as I appear as three different entities on IMDb because the filmmakers I worked for neglected to see if I was already represented and created a new page for me each time.

This is certainly the case with other actors/crew members, and could go some way to explaining the discrepancies you have discovered.

Bottom line is IMDb is not infallible, and it would indeed require some serious cross-checking with other sites in order to confirm the actor you are researching is the right one.