If you have a fully synced Bitcoin Core... and you load a wallet.dat into it (and it has been rescanned) and the balance is zero, then the wallet is, unfortunately, empty.
If you have transaction history showing, then you can probably trace where/when the coins were sent. If you have no transaction history (as well as zero balance), then the wallet was never used.
Just because you have a lot of old wallets, it doesn't mean those wallets actually contain anything.