how to meausre rfid tag signal
I possibly misunderstood the question. I thought it was about performance test of a given RFID system. If this isn't the case, what's your purpose of "measure signal strength of an RFID tag". Which information do you want to get from it? Generally, most RFID interrogators don't provide received signal strength information. They are trying to identify any receivable tag, up to the receivers noise limit, usually.
Because neither interrogator nor tag have omnidirectional antennas respectively coupler coils, field strength isn't clearly related to distance, which is probably the information you're looking for. From theory, circular polarized UHF antennas should operate nearly omnidirectional, but you also have to deal with multipath transmission in any real enviroment. So n-channel diversity would be needed for a reliable distance estimation.