One can know the contract's native balance by using
But, How can one know the native balance of a particular address/account in ink? For example, knowing the native balance of Alice, Bob, Charlie e.t.c?
I'm not sure of any specific command in ink! to fetch the native balance, but it sounds like you want to build a chain extension that can return the user's balances from the runtime. You would add the chain extension in the runtime configuration, for example: https://github.com/paritytech/ink/blob/80d302eb9b9cddb726200a9a86c71ae344d1b042/examples/rand-extension/runtime/chain-extension-example.rs#L20
Then you can configure the chain extension in your contract and call the function.