Get jumpers and run a lead from the battery - to a good engine grounding spot neat the starter.
I usually use the oil drain plug, if it is metal.
Turn the ket
Engine cranks = Bad battery ground .
No joy then put a jumper on the battery + and carefully touch to the starter + terminal
You will get a really big spark so be ready for it
Starter cranks = bad power cable or bad starter
No joy repeat with a known good battery .
Get back with the results
In the workshop I do this with the blower housing off so I can watch the starter pinion moving ( or not moving as the case can be )
