Combustion efficiency is defined by the pecentage conversion of the hydrocarbon components to their end products of combustion i.e.  CO2, and H2O.  If following combustion there are no remaining unburned hydrocarbons or CO present in the gas then the combustion efficiency of the  system can be considered to be 100%.   It is not practical to measure combustion efficiency of a flare as it by design an open  combustion system operating in a constantly changing environment.  In practice the combustion performance of a flare is instead assessed by its ability to flare smokelessly.