How to test pre-out voltage??

Read the AC voltage at the speaker output terminals on the amp, with the gain all the way down.  Have the engine running, and speakers unhooked.  If you read a two channel amp, a reading at each set of terminals is about half the preout voltage.  A reading at the bridged terminals is the full preout voltage.  At least, this is the voltage that the amp is getting. 

I have done this using an adjustable line output converter with a factory head unit, and I set the voltage output of the converter, in this way, to meet the maximum voltage input allowed by the amp.  But I haven't tried this using an aftermarket head unit. 

Use a test tone signal to get realistic results.  The meter can then also be used to set output voltage of the amp while adjusting the gain to a more sensitive level.

I don't know if there is a more accurate way, at the car.  But if there is a better way someone will post it up.   I know this, though.  Different test frequencies will yield different results.  That's why when setting amp gain you would play the range of frequencies that the given speaker is going to be playing...up to 100 Hz  for the sub amp, and mid and  med high freqs for the speakers amp.  That way you can find the loudest frequencies.  So it's good to have a full range of freqs to work with.