A sensor apparatus and methods for facilitating combustion of a gaseous fuel are provided. The sensor apparatus comprises a combustion apparatus defining a combustion chamber therein. The combustion apparatus is configured to combust a fuel-air mixture within the combustion chamber to produce at least one combustion product. At least one optical diagnostic apparatus is coupled to the combustion apparatus for measuring at least one property of the at least one combustion product within the combustion chamber. A controller is coupled to the at least one optical diagnostic apparatus, and is configured to determine the Wobbe index of the fuel in real-time based on the measured at least one property of the at least one combustion product and pre-determined combustion state data stored within the controller.