How can I validate that an XRP address is valid? An address like rhe8v7GW1jj4qTXsLF9fwFrum9oQD5VXG7 - how do I know if it's valid in the XRP network?
Ripple addresses, like Bitcoin and most other currencies, are Base58Check encoded. As long as an address validates against the base58check algo, it is a valid address on the ripple network.

Do note that it is possible to generate valid addresses without knowing their corresponding private key. You should take steps to ensure you have the private key for an address before sending funds to it.