The air/fuel mix and amount compression is far more of a determinate of how completely the mixture burns than the heat of the spark. The spark just starts the process, though a cooler spark lasts longer (measured in degrees of crankshaft rotation). A hotter spark can more easily ignite more difficult (leaner) mixtures.
The only thing you can measure at the exhaust is how complete the combustion is, and from that perhaps extrapolate the mixture if you know the other variables.
Air/fuel ratio would be determined from the jet size, air flow, altitude, barometric pressure etc., as well as oxygen content of the exhaust (that's how EFI does it).
