Is there a client API to query a Casper node's RPC endpoint?

You can query the JSON-RPC API of a node on a Casper network. You will need the IP address of a node and the REST endpoint for status and metrics, which is by default 8888 on Mainnet and Testnet. You can find specific node addresses for Testnet or Mainnet.

http://<HOST>:8888/rpc-schema

You can also run the Casper client list-rpcs command to get the full list of available JSON-RPC methods. You will need the RPC endpoint for interaction with the casper-client, which is by default 7777 on Mainnet and Testnet.

casper-client list-rpcs --node-address <HOST:7777>

account_put_deploy
info_get_deploy
state_get_account_info
state_get_dictionary_item
query_global_state
query_balance
info_get_peers
info_get_status
info_get_validator_changes
info_get_chainspec
chain_get_block
chain_get_block_transfers
chain_get_state_root_hash
state_get_item
state_get_balance
chain_get_era_info_by_switch_block
state_get_auction_info
chain_get_era_summary