Unfortunately most of us don't get to run DMX regularly either because we're working in mostly installed facilities or we can't afford intels. Due to the low demand and use for DMX, most of us won't be running it regularly. I'ts pretty much a set it and forget it type thing, so our DMX won't get much abuse. In conclusion, the actually need for a DMX tester is very slim. But, for the rare occasion it is needed, a regular meter will do fine. But I would be nice to have one to make sure a cable is suitable for digital signals and up to protocol standards. avkid, I've never thought much about these devices. Do know of any good units? I'm sure they're a bit pricey for high schools, being that a real DMX tester will transmit and receive a digital signal.
I used to use one all the time. But most of the lights we use now have a dmx indicator, showing whether or not that fixture is receiving data. 99% of the time that is good enough. About the only time I still need one is if I'm doing conventionals, but don't have enough to warrant bringing a dimmer rack with a readout, or if I'm using some of our ledwash lights, which don't have led readouts. However, those don't usually give me problems so I don't need to test them anyway.
Sending a signal down the line as opposed to just testing continuity can be a large difference between something that meters out correctly and something that has a shieldwire that's wired to the wrong pin or conductors that while they conduct still are damaged sufficient that they won't carry a signal. There are various forms of DMX tester on the market. On the other hand, some of the better grades of sound cable testers that will test between Low and high impudance will also often be sufficient to test the signal for ability to carry data signal.
In tester type there are some that need to test the cable from both it's ends - at times a pain in the rear, and others that will instead test the signal provided by the light board that can be of huge advantage while on a ladder with cable strung. A multi-meter, continuity checker or sound cable checker won't test other than both ends. There is some gear on the market that will read the DMX signal and show if it's good or not by way of either being it's own reciever or in being in-line and sensing the signal. Such is different equipment and very useful once installed but not of much use in pulling cable and pre-testing it for the gig. To be really able to test your cable, you need a way of testing it both as cable from both ends and as signal reader on the end of it once installed. Two very different items unless something that will do both.
The Ohm's meter will test for proper wiring, but unless you also un-screw the plug so as to ensure proper pin configuration, you don't know if it's the cause.
Other ideas. Never use other than solder connection plugs. Others will pull loose in sometimes being in contact and otherwise shorting. Always wiggle the cable near the plug while testing a pin. At times it will be shown that while you might get in general continuity, once you wiggle the wire it will in only being a single strand in contact with the other broken part of it will transmit the data while in the right position to touch, wiggling the cable will show problems when it is not at the right angle to touch.
When possible, avoid 24 ga wire. Too easily broken. Instead use 22ga if not 20ga conductors.