It aknowledges when you donate the last fossil. Do you have Archeopterix? that little b* is in the dino room, small and if you don't have it, you don't even know it's missing. D:
He starts dialogue after you've donated with: "Hoo hoo! Is it true? The final fossil has been found?"
If he hasn't given you that dialogue you haven't finished the museum. Look around carefully and see what you might be missing, also check your catalog and compare it to a list online. I have hella dupes so if you figure out what you're missing I'll be more than willing to give it to you.