How do I know whether or not I need to replace the flywheel or the starter?

What is your question?

Truck was making a new noise when I would start it. Suddenly wouldnt start any more.Was told its flywheel, or i need to tighten bolt on starter and all would be well. After he did that it would start. Then I got someone else to look at it. He aligned something the truck started. Drove it around the block, turned it off then it wouldnt start again. He said teeth on flywheel were worn down. So is it a starter or flywheel?