How do I measure flywheel to see which one I have??????

Flywheel out of what, a WWII Bell Airacobra?

Small block or big block?

Any chance it's a TRUCK flywheel

If this is a SB passenger car flywheel, it can only be a 10 1/2" flywheel. If you have a passenger car bell and the flywheel fits and the starter properly engages, it cannot be a truck flywheel

If it's out of a truck that's a different matter

If this is a BB passenger car, it could be 11" with 11" cover, or the 70/ later 11", that is, 10.9" which uses a 10.5" bolt pattern

If it's a BB truck, who in heck knows?