TOOLS OF THE TRADE!!!
First tool you should have is a DMM or DVM
There are many DVM/DMM meters out there that will work, you DON’T have to buy a super cool/expensive FLUKE. All you need is it to read DC volts and AC volts and you are good.
Second tool would either be a Oscilloscope or an RTA
The 2 scopes I like are TPI-440
Or the really small Velleman HPS140i
RTA (a must have tool for ANYTHING audio related)
A REALLY great HandHeld RTA is the Phonic PAA-3
And if all else fails -an IPhone (with a PAID Audio Tools RTA app installed) works pretty damn well!!
(get the low down on the application here – http://www.studiosixdigital.com/audiotools/)