Install S&S ProTune II on a laptop and plug it into the ECU diagnostic socket. It will tell you what current and historic fault codes are there and what they are for, it should even allow you to clear them. As it's a simple check on the dash, voltage out of limits will give you a fault code, should be about 14.3v running.
I have ProTune on my laptop , and its at work ! - doh I will do this tommorow night , cheers !
