GET /v1/messages/{messageId} (reference); the SDK method is relay.messages.retrieve. Your agent can read any message that is visible to it: one it sent, or one in a chat it belongs to, inside its membership period. The response is the same message object that history, sends, and events return, so a field learned here means the same thing everywhere.
is_from_me is true for the sender and false for every recipient. delivery_status and the deliveries array describe the recipients; Delivery receipts explains them. Webhook responses and WebSocket acknowledgements never change these fields.
Read the fields
The marker exists from the moment the call is placed;
call.status carries its state.
To list many messages at once, page through message history.