Discussion: In addition to the normal troubleshooting tools like scope probes, oscilloscopes, spectrum analyzers, and other common measurement hardware, I also have favorite items I use frequently in my work. Figure 1 shows some of my favorite troubleshooting tools consisting of a pair of matched F-33-1 current probes, an F-65 current probe, different sizes of square wire loops, and a roll of copper foil tape, actually "Slug and Snail" tape from the hardware store.

The F-33-1 current probes are very sensitive with a transfer impedance of about five Ohms, that is five Volts/Amp output into 50 Ohms, with a flat frequency response from several MHz to about 300 MHz. They are ideal for measuring common mode currents to predict radiation from system cables. You can read more details on measuring common mode current to predict emissions in my March 2006 Technical Tidbit: Predicting Cable Emissions from Common Mode Current.

The F-65 current probe has a one Ohm transfer impedance, one Volt/Amp, but has a flat frequency response from one MHz to one GHz making this probe ideal for time domain measurement of impulses like ESD using an oscilloscope. One such use of the probe is illustrated in my March 2011 Technical Tidbit: Using High Frequency Measurement of ESD Current to Find Problems with an ESD Simulator.
Figure 2 shows a tip that helps when using current probes for impulsive currents. I label the probe for the direction of current that results in a positive output voltage from the current probe. This trick helps avoid a lot of turning the current probe around when making impulsive measurements and in some cases in frequency domain measurements as well. In Figure 2, I have labeled the direction of current that yields a positive output on paper tape attached to the probe.

In the upper half of Figure 1, you can see three square magnetic loops made from stiff brass wire and covered with heat shrink tubing. This may be the most useful tool I use! This website is filled with things you can do with such loops from measuring high frequency noise to injecting pulses into circuits for troubleshooting.

Of course, the copper tape shown in Figure 1 is probably in many engineers' tool box. It is useful for modifying circuit cards, shielding, and many other uses. One use is to wrap a cable with a short length of copper tape and discharge an ESD simulator to the copper tape to inject ESD like current capacitively onto just one cable at a time. This technique is described in my January 2009 Technical Tibit: Injection of ESD Current into Cables Using Capacitance. I like to use "Slug and Snail" tape from the hardware store as it is inexpensive and readily available.

The small yellow caps in Figure 1 are actually vacuum port caps for automobile intake manifolds, but make great BNC protectors at very low cost. More information on protecting BNC and SMA connectors this way is in my December 2008 Technical Tidbit: Inexpensive Protective Caps for SMA and BNC Connectors.