AGC circuit measurements , spectrum analyzers

What are some common measurements you do for AGC circuits? using a power meter and spectrum analyzer?

AGC circuits have multiple stages of gain ,

An AGC circuit has a feedback loop which feedbacks the outputs voltage and gain back to the input circuit. What is this feedback circuit called?

What is an AGC circuit used for? to do what? to amplify RF signals

AGC stands for automatic gain gain control or some times ALC automatic level control. You measure it with a signal generator and some sort of meter which measures the output. For the most basic case (AM radio), you would feed a modulated RF signal into the aerial input and measure the audio output. Then with the audio measuring meter left alone, change the attenuator on the RF signal generator. What you will find is that the audio output increases as the signal generator increases from zero to 50µV, then as the RF is further increased, the audio level does not change by very much, say over the range of 100µV to 5 mV the audio may change by 2 dB.
That is the basic system, for other sorts of modulation, such as FM, the AGC starts to act at a much higher level and is designed to maintain the linearity of the front end stages in the presence of strong signals. That is because the level of the audio does not vary with the level of the RF but only with its frequency deviation.
Frank