Measuring RF transmitted Power
You need


A RF anechoic chamber (wedges on the walls, floor and ceiling to prevent any reflections)
A test device with an antenna and physical setup (construction, mounting, form, shape etc.) that's very close to what you want to ship
A calibrated measurement setup, that's typically multiple receive antennas in the chamber or a full 3D setup (vertically movable antenna plus a horizontal turn table)
Very good understanding of the polar pattern of your transmit antenna
A good calibrated spectrum analyzer
Time &amp; patience


This is not an easy measurement to make, so most companies farm that out to certified test houses, who have the equipment, facilities and experience to do this.