Pancake Coil Magnetic Field Strength
You definitely can do it! If you have an oscilloscope you can make quite accurate magnetic field strength measurements. Here is one paper, in which the procedure is explained and the properties of the probe antenna are also explained (page 4 to see the equation).

You just have to be careful which kind of probe you are using, so the cable doesn't capture power, only the loop. If you have a low frequency you can use twisted pair cable with a loop in the other end:



Do be careful to make the probe small enough, if it is too close to the antenna conductors the results will not be accurate.