OK, using a floating voltmeter, plug into the jacks on the side of the head and read from black to red, then blue to yellow. You press the switch to get a reading.
Depending on the head cal, its either .1V per amp or .2V per amp. Light voltage can be all over the place, with no specific cal.
DO NOT CROSS THE STREAMS< IE DO NOT CROSSPATCH THE LIGHT SIGNAL TO THE CURRENT SIGNAL< IF YOU WANT YOUR PSU TO LIVE. THAT IS WHY THE BUTTON IS THERE>
THE so called overcurrent led is just a led across a resistor in series with the tube. It is a NOT a true measure of overcurrent, in a photocopier running at 23.7 mW, it was a indicator of tube lifetime. In lightshow service, it just serves as a eyeball way to check current.
As for the power drift, you may need to touch up the resonator tuning. The laser wants to see heat pulled out of its baseplate. If you run it without it setting on metal, the tuning can drift a bit. That can explain your loss of power. Also running a laser at higher currents can release 20 year old crud into the optical path. I would not worry just yet. Keep it down at 8 amps or so...
We have plenty of stuff on tuning in the FAQ, both Sam's and My versions of how to tune.
Steve