Digital Multi Meters- you need one, you may just not know it yet, these things are your eyes into the electrical circuit.
Back in the old days, folks used analog meters (those rascals with the needles that read wrong if you looked at them crooked) to measure volts, amps and ohms with each quantitiy requiring a different meter, but, alas, with today's new technology, they've wrapped all three meters up into one with a digital display, thus the term digital multi meter. Analog meters still have their place and are great for some applications but they are not as accurate and are more intrusive in the circuit being tested. More intrusive in the circuit? What the hell are you talking about? The analog meter requires a bigger taste of what's happining in the circuit in order to take it's reading, in a computer circuit that's dealing in Milli amps, this can be fatal to the computer. A decent DMM just needs a small taste and doesn't disturb the circuit as much, therefor is not only healthier for the computer, but, is also more accurate.
DMM's range in price from $10 to over $400, a real cheap model is just that, cheap junk. On the other end of the cost spectrum are models with more functions than the normal human will ever figure out or need. A good DMM can purchased for $40-60, the Sears Craftsman model 82040 is a fine unit at a reasonable price, Radio Shack also has some good values on DMM's. When shopping for a DMM, make sure it has the ability to measure amps through an internally fused circuit (most have 10 amp fused capability), dont forget to buy extra fuses for the durn thing. Most meters are auto-rangeing, meaning the meter will fing the correct scale for the display. This is a good thing BUT you must make note of the units being displayed when you take your reading