Hi, Mandi - are you asking about provenance or attribution? I'm guessing if the work has been in the family since before your father's death, chain of custody would be fairly easy to prove.
On the other hand, attribution might be a bit more difficult. Did he work with others? Have a unique style or technique? Were materials (especially supports - canvases, paper, etc.) left in his studio when he died? If they match the materials of finished, unsign works, this could help to attribute.
Were the dioramas created for institutions where purchase or commission records would still be available? Did he purchase art from others, or did he and his colleagues/friends trade works? work in similar styles and methods? This could muddy the waters.
If he had a coterie, its members might recall his working on certain pieces, or even better, have photographs of him and the works in his studio. Artist friends might recall trading or working with him.
Good luck. Vivian