Your coil might be capable of generating 6K volts.. but if the spark plug sparks at say, 2000 volts, thats all the coil will put out. In other words, the condition of the spark plug, it's gap and it's cleanliness and it's sparking environment inside the engine determines how much voltage the coil must generate to arc across the plug gap. A coil produces just enough voltage to arc across the gap.
If your plug is dirty or there are other reasons high voltage is needed to arc the gap, the generated voltage required of the coil might be 10s of thousands..
It's not a good idea to force the coil to produce these very high voltages. Voltage will build until something, somewhere arcs. It might arc internally in the coil, and damage wire insulation or similar. This damage builds up and the coil may develop a new arc-pathway that has a lower voltage than your sparkplug gap. Meantime, the engine misfires more often as the problem gets progressively worse.
If the coil's wiring shows continuity where it's supposed to and if the coil is well grounded where it should be, that's about all you can do to test it with common tools. You'd need the coil's resistance specifications and a very sensitive ohm meter to do more.
If, after a basic exaination of the coil and all other ignition components you suspect a bad coil, the best test is to replace it with a known good one.