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 