How do you determine what animals a Druid has seen prior to the start of his adventuring career?
With an ability check.
Player: I definitely saw a giant octopus one time when I went to the beach!
DM: Maybe you did and maybe you didn't. Giant animals are pretty rare, and it says here that you grew up in the woods. Show me a Wisdom (Survival) check, DC 15, and we'll say that you were just at the right beach at the right time...
In general I find ability checks, with reasonable DCs, are a great and rapid solution to this sort of DM-player dispute. "Can cast the spell just right so it doesn't hit my allies?" "Do I know anybody in this town?" "If I cut up the carcass for parts, are they worth anything?" ...Make an ability check!