You need an ammeter capable of measuring 150-200 amps. The tester I used was actually an alternator output tester, but amps is amps.
Take the positive cable off the battery and put the tester between the terminal and the cable. The tester becomes part of the circuit.
Crank the bike and read the amps.
In these modern times there are inductive ammeters that simply clamp onto the battery cable and measure the current through magic and the force, no disconnect/reconnect of cables necessary. You could use one of those, too.
I am told that the starter should draw no more than 75 or 80 amps while cranking the engine. Like I said, mine pegged my 160-amp tester, so I knew I had an issue.
You can test the starter on the bench as well, just bolt it down (its torque will send it flying if unrestrained,) apply power and let it spin a couple of seconds. Should be less than 30 or 35 amps spinning free like that. Mine was nearly 90, confirming that I had a problem.