If your central is an iOS app and you are connected to a BTLE device (peripheral) you can use this:
As type you use:
CBCharacteristicWriteWithResponse (to get the response)
CBCharacteristicWriteWithoutResponse (no response)
If you choose the response you will get a callback:
then you can check error value:
if (error == NULL)
See leatt_regWriteRspCb() and how hello_client sample app uses this. The callback registered with this function will be invoked when the write is executed successfully by the server.