For an FA2 nft contract, how can I get the address of the owner of token_id 0 from inside another contract?
The specification does not enforce an implementation for this. You can only verify if a given address owns the token by calling balance_of callback view. In the case of an NFT, balance
1 would mean that a given address owns that token and a balance of
0 that the address does not own it.
Check this thread for an example on how to do that.