You answered your own question... "I have a function that returns entirely different objects based on certain cases."... Why would you want to have the same function that returns completely different objects? I can't see a reason for this to be useful, maybe you have a good one, in which case, please share.
EDIT: Considering your clarification, you should indeed use a marker interface. "completely different" is quite different than "are the same kind". If they were completely different (not just that they don't have shared members), that would be a code smell.
