Measure AC magnetic field strength between 100kHz and 300 kHz
As with any coil, induced voltage is N\$\dfrac{d\Phi}{dt}\$. 

So, armed with the number of turns (N), the voltage, the frequency and the area of your loop, you can calculate the average magnetic flux density seen by the coil.

The coil should be measured open circuit and the fewer turns the better because coil parasitic capacitance can easily resonate the circuit and give big errors.