The most accurate test for flywheel diagnosis is the traditional age old hard way. Take the inspection plate off. Disable the ignition. Mount a dial indicator and put a wrench on the crank pulley or use a flywheel pry bar to rotate a full revolution and consult a flywheel manufacturer specsfor runout tolerances. 

Often times, just a visual inspection through the starter hole, after starter removal, will show worn or missing teeth. It is imperative to chalk mark the flywheel to ensure a full revolution of visual inspection. The flywheel will have favorite places to stop when the engine is killed, due to timing and compression. These 2 or 3 spots will have much more wear on the teeth because the starter bangs on the ring gear more frequently in these areas. The rest of the teeth may look perfect, but the flywheel is junk. 

Ever encounter a car that the starter grinds sometimes, then sometimes it works perfectly fine? Would that be a bendix acting up sometimes ? or the flywheel ? This method helps

figure that out. 

Test the starter several (or many) times until you find the spot that grinds. Mark the crank pulley with a piece of chalk and remember where it was or make 2 corresponding marks that line up on the crank and some surrounding piece. 

Now begin starting the car again. Each time the starter has a grinding problem,

mark the spot with the chalk as done previously. Flywheels will have about 3 places

they normally come to rest, because of timing and compression. Does the grinding sound

seem to occur on or near the chalk marks? Did the grinding start to develop a pattern?

You just diagnosed a worn flywheel without ever touching a wrench.

You know from this test that a simple starter replacement will most likely not solve the problem. Refer to article title “Putting a Band-Aid on a Flywheel”