which tester do you guys use?

I use those $4 test lights from walmart most of the time. That's more for reasurrance rather than testing though. Just double checking wires in cars I already know fairly well or ignition wires. I don't blindly poke wires with it most of the time. In that case I use my computer safe test light AKA DMM (Digital Multimeter). Honestly I got in the habit of using the test light all the time and even though I didn't fry anything using the test light, I'm working my way back into the habit of using the DMM more just to be safe.

DMM, even the radio shack one for the "throw away" use.   You know you'll end up using it later for other stuff.   Just say NO to test lights !

 I use a DMM  it's not cheap but very useful.If you bui one be careful that the measurements must be maximum 0.5 seconds .

I use a device called "THE PROBE". I bought it from Matco tools for around 60 dollars. Its an led test light that plugs into the cigarette lighter and has both a red and green led on the shaft. If you probe a positive wire, the red lid lights up, if its negative, the green lights up. Plus the tip is super sharp (beleive me, its poked into my finger a few times) and it comes with an extra tip, a cap to cover the tip, and it all fits into a plastic blow-mold case. It is by far the best test light I have EVER used!

dmm are easy to use for basic voltage readings just set your meter for dc volts always set it to the higher voltage then what you are testing (example)you are testing a 12 volt bATTERY set your meter to 20vdc and then test. Heres how you test put your black lead to ground and probe the wire you need if it reads 12 volts then this is a positive wire,(example)you probe a wire at the ignition harness with the red tip and you see no voltage with the key at rest now you turn your key to the acc position and now you see 12 volts well this is a positive wire and this would be your accesory wire in the vehicle. Now heres how you would test say a negative trigger door lock sytem this time put your red lead to a known positive voltage source now you will probe the wire with the black lead if the wire reads 12 volts this is a negative wire. Basic voltage tests are easy but you will have other things you need to know your best bet would be to go to home depot and pick up a multimeter book they are like 10 bucks this will show you everything you can do with a dmm.

continuity can tell you if your wire is broken or if it is a good connection all you have to do is set your meter to resitance or if your meter has a continuity function set it to that now take one probe and poke one end of a wire and then take the other probe and poke the other end of the same wire---if the wire is not broken or is complete you will get a beep sound out of the meter. You can also use this to test for a short to ground by probing the wire you think may be shorted and put the other probe to ground if the meter beeps you have a short. Now above i have said listen for beeps this is only if you have a meter capable of an audible continuity test,but if not just have your meter set to dc resistance and look at the meter if it shows nothing or 0 you have no continuity or a broken wire, it should show about.02 on a wire that has continuity or is not broken. Basically if you see the meters screen change to a number you have continuity if it does nothing you dont .

actually, "The Probe" is only $50 right off Matco's website.... but i guess once you add shipping, it's close to the guy-driving-around-in-the-Matco-price excellent recommendation btw, most valuble tool in my toolbox for sure Ya gotta have stuff that sucks to have stuff that's cool - Butthead