dm648 jpeg encode

I‘m using the jpeg encoder library ,input format for YUV422I (dynamicParams.inputChromaFormat = XDM_YUV_422ILE),and output format select (YUV420PJpegParams.forceChromaFormat       = XDM_YUV_420P) ,after the jpeg encoder the output file is  JPEG,but How can I verify that the output format  is YUV420P? whether the  input data  in accordance with the output format will be re-arrangedand then encode it?