If your crankshaft pulley is loose you can know by the issues that will occur which include irregular idle , engine vibration and even the belt slipping or wobbling and the pulley can slap up against something next to it.
If your vehicle has a harmonic balancer that is a rubber dampened pulley you should look for signs for wear and tear on the rubber like cracks, tears or drying out.
You can also take the belt off the pulley and try moving the crankshaft pulley and if it wobbles back and forth then it's loose and needs to be tightened or replaced.
A loose crankshaft pulley can also cause the drive belts to whine or squeal, especially when you accelerate under load.
Other signs of a loose crankshaft pulley are hearing rattling noises from the engine or chattering noises, most particularly when idling of the engine or even during cold starts.
The crankshaft pulley should not spin freely by hand when the engine is off.
The crankshaft pulley should be able to rotate but should also be difficult to turn, especially by hand.
You should be able to use a socket and ratchet to turn the crankshaft pulley and it the crankshaft pulley does not rotate at all then there's an issue with the engine.
If the crankshaft pulley rotates freely then there's an issue with the engine.
The crankshaft pulley is connected to the crankshaft which also drives the pistons and rods in the engine and so it should be difficult to turn.
The crankshaft pulley also drives other engine components such as the power steering pump, alternator and water pump through the serpentine belt.
When the engine is running, the crankshaft pulley also rotates and this same rotation is transferred to these other accessories.
The crankshaft pulley should spin easily when the engine is running but not when the engine is off.
If the crankshaft pulley cannot be turned very easily or at all it could mean the engine is seized or something is locked up in the engine or the pulley is misaligned or there are worn components.
Other pulleys such as the water pump pulley, alternator pulley, and power steering pump pulley as well as idler and tension pulleys should be able to turn freely when the belt is not in place.