by Biff44
I assume you mean an RF magnetic field? A DVM will not respond in time.
You need some sort of coil that will pick up magnetic fields. If it is a fixed frequency you can tune out the coil to resonate at the frequency of interest, making the job easier. Then either hook up the coil to a spectrum analyzer, oscilloscope if the frequency is low enough, or a diode detector/log amp depending on the recieved power. You can rotate the coil to figure out the x,y,z component.