power amplifier measurement

should we use spectrum anlyzer to measure the output power, gain
or should we use power sensor, power meter to measure the output power.
How to measure DC power to calculate pAE

Spectrum analyzer, power sensor or power meter, all are fine to measure the gain power, from the moment they are well calibrated. For spectrum analyzer setup beware choosing the right RBW and the right front-end attenuation.
DC power can be measured using a simple current meter, or a DC power supply which have an internal current sensor. One of the best on the market are made by Keithley
http://www.keithley.com/