The OPC specification has error and success results defined for the Write operation. If the PLC protocol supports a write acknowledgement, the OPC server will interpret it, and when the acknowledgement is negative or is not reeived, it can indicate a problem to the OPC client. Of course, if the protocol does not support a write acknowledgement, such indication is not possible,
It is also a common practice to subscribe to the same item, if it is also readable. A successfull Write will eventually be reflected in the new value of the item, sent by a callback to the OPC client.