Earlier this year, I published a tutorial on current probes and their application in Interference Technology (3/20/2012). The discussion included how to make your own probes from common ferrite cores, as well as commercial probes and how the DIY probes compared to the commercial ones. I finally wrapped up with how to use current probes to assess common-mode currents in I/O cables and how to predict pass/fail for radiated emissions by knowing the CM current on the cable.
Simple current probes made from clamp-on ferrite chokes.
design-4-emc.com 
I’ve been using this sort of probe for some time and a couple of manufacturers for the Amateur Radio market have sold current meters using these parts.
However, we need to remember that the common “43” material core as shown has about 100 ohms of impedance at 100 MHz in the conductor it is placed on, and will affect the actual current in the wire.