Standing waves along a parallel pair of conductors. You bridge the two conductors with some sort of voltage/current indicator (very simply a light-bulb with two

Standing waves along a parallel pair of conductors. You bridge the two conductors with some sort of voltage/current indicator (very simply a light-bulb with two wires sticking out of its base) and measure the half-wavelength in meters between two maxima. Note: to keep things compact, you use this method for frequencies around 50 MHz and above. I was shown this method when I was playing with valve/tube oscillators way back when I was a teenager. The other method is to compare the frequency as measured by your oscillator-style frequency meter (using known calculated values of coils, crystals and capacitors) against several known 'standard' frequency sources.