How to know balance in Ledger Nano S?
You may use a blockexplorer e.g. https://blockchain.info/xpub/${xpub} (replace ${xpub} with that string beginning with "xpub") to look up the balance on the web or use a wallet like e.g. mycelium.

You can look up this key as well as the private one with e.g. https://iancoleman.github.io/bip39/ – use a copy offline for your actual backup phrase

Have a look in BIP32 and 43 to learn about xpub and xprv for HD wallets (and later on BIP 39, 44 and 49).