How to tell when a ballast is going bad

I have two ballast a 400hps i built from a old parking lot light 12 years ago, I also have a sun system 430 watt that i bought 5 years ago. Hung both of them together the other day as i had more plants then the one was going to cover. When i turned them on i notice one light was like 30% brighter than the other one so i thought i'd chill for awhile while they got to charge all the way. When i came back half hour later it was still the same diff. So i turned them off and switched bulbs. After they cooled down i went ahead and switched them and the same thing was notice. It seems that my old homemade ballast drive my bulbs better than my sun system ballast at 430 watts. Any way to check the output voltage coming from the ballast?

volt ohm meter.

You know i believe that the output of the ballast with the capacitor is geater than 2000 volts.

briefly yes. don't lick the wiring.
compare.