To measure the RF power in EMC testing, there are a few things one should take care about:
Normally an EMC test system is not a well-matched RF system. Hence, is it a good thing to make a difference between the forward and reverse power. Another thing is: common RF power probes are only capable to measure power in a range from 100 pW up to a few Watts.
For those reasons in the EMC one usually uses directional couplers. A directional coupler has two separate outputs for forward and reverse power. There is only a small part of the passing power coupled into these outputs.
Directional couplers have a coupling attenuation reaching from 20 dB up to 60 dB.
The actual power measurement is done with a RF power sensor (which is connected to a power meter). Common power sensors are diode sensors (peak value rectification with following voltage measurement) and thermal sensors (measurement of the temperature rise of an electrical conductor).
In the feed-back control system shown above the directional coupler and power meter are used to measure the actual value of the forward antenna power. That forward power is a measure for the RF fieldstrength.