Each stage s-parm's as a function of frequency, time, temperature and voltage , then verified individually with test coupons ( small PCB's) with controlled impedances and pre-determined matching and taps to 50Ohm output SMA's for example to a Spectrum Analyzer with synchronous RF sweep generator or better a VNA. THe coupon test fixture must be calibrated adequate S parms when the circuit is bypassed and compared with circuit installed. Then interactive effects from mismatched stages must be tested.
With a good VNA and test fixture, one can calibrated a fixture to >40dB RL and 0.1dB insertion loss up to the desired GHz range. THe Anritsu VNA I Worked with using a customer made leadless chip carrier fixture for a customer chip had a flat response and accuracy of 0.1dB and a resolution of 0.01 dB up to 6GHz where Tx loss must be low and every 0.1dB for a Sat Phone means a compromise of range or harsh weather.
I setup automated tests for 3IP, s-parms, filter ripple, BW, skirt rejection, Group Delay ripple, etc etc. for the Iridium Satphones for Motorola & C-MAC