I'm pretty sure that as soon as the starter switch is pushed, the battery voltage will go to zero, or close to it, even if the engine won't turn. Doing a cranking test with the headlights on is another substitute for real test equipment. If the lights dim badly or go out altogether, the battery is discharged or dead. A quick bench test of the starter clamped in a vise with a 12V battery and jumper cables will tell if it's capable of cranking the engine, also.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!