You have to find out (from the documentation) where to find the status of the relay.
Depending on the device it can be discrete input, input register or even holding register.
In general, no way to tell.
EDIT: for example: with my PLC, I can read the status of the output by reading the coil. I can also control the coil by setting (writing) it. With some devices reading the status is separated from command – this is your case.
EDIT2: I tried searching for “relay” or “reel” in the documentation but found nothing. Have a bit hard time deciphering this device myself.