You can get a quick idea of battery condition simply by pushing the starter button. If the motorcycle turns over and starts battery is good, or good enough anyway to not be causing this condition. All I know for sure is one time on my 1978 KZ650 I was riding along just enjoying the ride and noticed my turn signal indicators on the dash were not working, then I noticed I had no signals or brakelight or taillight or headlight. I turn off the key and start investigating and find one terminal has broken off the battery. Every bulb that was used from then on out was burned out, 10-15 bulbs and I probably only rode without a battery for one mile. I'd guess the voltage was up over 20v, but no way to tell.
321,000 miles on KZ's that I can remember. Not going to see any more.
