If I understood correctly, you are trying to access the value of the string descriptor which you have set already. In that case, before calling the CyU3PSetDesc API, you can take a back up of the value which has been set using that Descriptor.
Please note that CyU3PSetDesc API only stores the string descriptor in an internal structure. It doesn't yet send the descriptor to the Host PC. Only after the CyU3PConnectState is called, the descriptor is sent to Host.