I'm working on a distributed I2C bus system that uses standard telephone cable and RJ11 jacks for interconnection. Plug and play - if you get the software right. That requires cables that are correctly wired. No shorts of course. But also no twists. Five volts should not appear on the ground connection. And vice versa. To do the testing I have designed a board with three resistors and six LEDs. I use a six volt battery pack of 4 AA cells with a built in switch to power the tester. The batteries should be good for about 120 hours of continuous use. If the cost of batteries irks you the tester can be powered by a 5V wall wart supply. Or a convenient bench supply. The voltage is not critical. Anything from about 4 volts to 6 volts will do. Absolute maximum current possible is about 35 mA at 6 volts. Normal current with a working cable plugged in and a 6 volt supply is about 25 mA.
In use the supply polarity doesn't matter. If you use the supply conventions shown the green power good light comes on with power and if the cable is good you get two green lights in addition. If the cable is reversed you get a green power good light and two red lights. If you reverse the power supply red will be the power good light and two additional red lights will indicate a good cable. Various other faults will give you different light combinations which you can work out from the schematic. You will also find the parts placement and parts list with the schematic.
The only thing that needs special attention during assembly is LED orientation. I used a 5V supply and some clip leads along with two 150 ohm resistors (one for each clip lead) to probe the LEDs before mounting them.
The boards (I2C Simplest Cable Tester 31 Oct 2012) are available from OSH Park for $2.45 each plus shipping.