This is one of the simplest ways to measure an RF output. You just take the input, step down the current that can be transfered to the circuit by a small value capacitor, which will act as a high impedance path for the RF signal ( you can use a resistor as well, but capacitors offer DC decoupling which is sometimes needed). Next step is to rectify the signal and feed it to a DC amplifier (the transistor at the right). You can use a more accurate topology like the one with the transistor at the right, where using a voltmeter shows you more accurately the output level of your RF amp. This way you can tune the output filter more precisely until you get the highest reading on your voltmeter.
