How do you use an oscilloscope to detect radio waves intercepted by a transistor radio?
I would be less concerned with using the resistance of different materials for the reason of showing the power level of RF.  You could build a simple crystal radio with a tank circuit to show resonance which is how you would receive the power of a radio wave to begin with.   You could get creative and make a simple crystal radio  where you replace the diode with a transistor, then use a second tank circuit to tune in a close by, strong station to bias the transistor.  This will show the power of an RF signal.  You would naturally do this in the broadcast band.