You need a signal source to generate RF in the proper range and detector to attach to your FSS. The detector may be as simple as a diode or may be a spectrum analyzer. Without more detailed information about what it is exactly that you want to measure, it's hard to make further recommendations.
you need two horn antennas of same specifications. one antenna is fed by the microwave source and act as transmitter. another antenna will be connected to a power meter with a power sensor. now measure the power two times- first without FSS between the antennas and then with the FSS placed between them. now get the difference and you will get transmission in dB.