Don't know for sure, but I'd guess any of the following
Clutch disengage/engage problems
Excessive vibration while the car is not rolling
Strange noises while starting or shutting down
Rattling of the loose bolt inside the bellhousing or pressure plate (if it came all the way out)
Someone posted a while back that a test for a loose flywheel was to rev the engine and turn the key off while the car isn't moving. If you hear a "clunk" that means the flywheel is loose. Dunno how true it is.
Not sure if the flywheel came loose but I had a flywheel bolt fall out while driving. No vibration or anything until I heard a noise like a machine gun firing and a sack full of aluminium cans dragging under the car.
Bolt was flung around until it punctured the bellhousing,shooting the starter motor on exit.
What are the symptoms of a loose flywheel? 
