There are a several variables/calls that delivers specific information:

1) LibRaw::imgdata.idata.dng_version - 0 for not-DNG, non-zero for DNG files
2) LibRaw::get_decoder_info() call
3) four calls:
int is_fuji_rotated();
int is_sraw();
int is_nikon_sraw();
int is_coolscan_nef();
and protected
virtual int is_phaseone_compressed();
Will return non-zero for some specific cases that might be interesting in processing workflow