I was recently troubleshooting an ignition problem and wound up buying a little tool for estimating spark voltage from Autozone. It's basically two pointed screws pointed at each other. You adjust one screw to change the gap and there are some markings on the body of the part to show what gap size corresponds to a particular kV value (so if you can get the spark to jump 0.25" then that is roughly some voltage value).

It cost about $5 and would give you a rough estimate of the voltage reaching your plugs.

If you have the equipment, I think you could pull the coil and send a low voltage, low frequency AC signal into the primary and then measure the resulting voltage on the secondary to determine the ratio. You're trying to get at least 20kV from 12V so the ratio is going to be big. You should note that if you change the ratio to get more voltage, you're going to get less current (you can't create more energy), which might be bad.