How do you know if the flywheel is bad on a 2000 Chevy Blazer?

If the ring gear is damaged the starter can't engage.

if the crank position sensor cannot see the flywheel then it doesn't communicate with the computer to send the spark to the ignition spark plug, as well.

If you have broken or missing teeth on the flywheel ring gear the starter will just spin or make a horrible clunking noise. You can remove the flywheel inspection cover to get a look at the flywheel. Hope that helps! Jim

You can look at it.