The average oscilloscope probe has an earth lead with an alligator clip on the end (My Tektronix ones do). Get this alligator clip and put it on the probe tip. What you get is a loop antenna - albeit not too effective. Turn up the vertical sensitivity on the oscilloscope and use this loop antenna to sniff out electromagnetic signals. As you move this loop antenna near wires that are carrying high frequency signals, you will see the evidence on the oscilloscope screen. It's crude but can be very effective.