you need to know the rated wattage output of the bike (should be in the service manual)
then the wattage of everything running (lights mostly.. everything else we can guess reasonably accurately, but if its in the manual post it)
then the wattage of the lights. and do the math to see how far over you are.
although i would use a meter and test that way.. at the very least, use a voltmeter and see what the power drops to...
ex:
say, if you have the bike running at cruising RPM and it makes 14.0v... and you turn the lights on and its still 14.0v, then you're perfectly ok.
if you have the bike running at cruising RPM and it makes 14.0v... and you turn the lights on and its makes 13.0 - 13.5v, then you're getting close to pushing it, and id check the stator output wires carefully. for awhile, especially their connections but you should be ok. the battery is still charging when you're moving.
if it drops to battery voltage (12.5ish) or lower... then its far too much.
thats just another way of doing it... probably the easiest.