Not sure if I fully understand your question. The function you have written will already take any of the pre-defined projected coordinate systems defined by the enumeration 'esriSRProjCSType.
If you asking about a way to determine the coordinate system of the X and Y parameters of your method 'GetLatLongFromPlannerXY' then the answer is no. The actual XY values could be from any projected coordinate system. The are simply just numbers. Presumably thru user input your code would determine which projected coordinate system the XY pair represents and then your method can convert those values to their geographic (longitude/latitude) coordinate values.
DG