Pros: easy to read, easy to use
Cons: doesn't test GFCI outlets, cheaply made
Owning an old house can be an educational experience, and the level of education you attain is usually in direct relationship to the age of that house. Given that mine was built before 1900, you can guess that at least one previous owner learned "on the job" while remodeling, upgrading, or just plain maintaining the house. Over the past few months, I've determined that one such on-the-job training session came while a Previous Owner (termed a "PO" by old-house buffs) was working on the electrical system. To be sure, there were clues - little things like noting all the three-prong plugs have been installed upside down; bigger things like finding that twisted-pair lamp cord had been used to connect light fixtures instead of 12- or 14-gauge copper jumper wires. I'm pretty sure a building inspector never saw that!
One simple tool that I've put to good use recently is an electrical little circuit tester. Mine is a GB GRT-500, made by Gardner-Bender. To use such a device, which looks like an oversized yellow three-prong plug, you simply plug it into an outlet. Plugging it into a power strip or extension cord also gives a reading - but it's accurate only if the extension cord has a three-prong plug. As long as the outlet is "hot" (meaning that there's current running to it), one or two lights will flick on immediately. Assuming the plug is right-side up you merely read the code from a sticker on the body of the plug to determine the state of the outlet. Given that every connection has (or is supposed to have) three wires - hot, neutral, and ground - there are six possible results:
Open Ground: the ground wire (the naked one) isn't connected to the plug, or the ground circuit is broken somewhere in the wiring.
Open Neutral: the "return" portion of the circuit (the white wire) is not connected to the plug, or the circuit is open somewhere in the wiring .
Open Hot: No lights at all means that there's no power to the plug, probably because the hot (black) wire isn't connected. Who knows what's going on with bare and white. You'll also get this if the circuit breaker is tripped or the plug is controlled by a switch that's in the OFF position.
Hot / Ground Reversed: The bare and black wires are connected to the wrong terminals, white is correctly connected.
Hot / Neutral Reversed: The black and white wires are connected to the wrong terminal.
Correct: All three wires are connected to the right terminals. Whoopee!
The more serious problems - Hot / Neutral reversed or Hot/Ground reversed - are indicated by a red light; the other conditions only cause amber lights.
Hot / Neutral reversal, called reverse polarity, is probably the most common error you're going to encounter. While few (if any) electrical devices care much whether the current runs left to right or right to left, almost every electrical plug made in the past few decades is polarized for safety's sake. They can only be plugged into (modern) outlets in one orientation, which is designed to ensure that if there are any electrical shorts in the device, exposed metal parts don't carry the current. Wall sockets that have reversed polarity are considered an electrical shock hazard, which is why they're noted on pre-purchase house inspections. Fortunately, the condition's fairly easy to fix: all you have to do is reverse the black and white wires.
This particular model doesn't analyze GFI (ground-fault interrupter) circuits, which are supposed to be installed in wet areas (bathrooms, kitchens, garages, basements...). Gardner-Bender does make a tester that does so, and sells it for about half again as much.
Since it's bright yellow, the tester is easy to keep track of. The only problem I've had with it so far is that the prongs seem to be a little closer together than normal, which makes the tester hard to worm into sockets. The little guy comes on a blister card that also has the light codes printed on it, which proves useful if you've bought a house wired by someone (a PO) who thinks that the ground prong on a three-prong plug is supposed to be at the top. Idjit!
In case you wondered, about half the plugs in my house are / were reverse polarity. Many of the rest aren't / weren't grounded. I repeat, "Idjit!"