Examining the obs_seq.final file
If you are testing with a single observation, just look in the file. If this file is in binary format, edit the
&obs_sequence_nmlnamelist in input.nml so the output observation sequence file will be written in ASCII:
&obs_sequence_nml write_binary_obs_sequence = .false. /
Then rerun filter to regenerate an obs_seq.final file in ASCII. For an explanation of the contents of your obs_seq.final file, see Detailed structure of an obs_seq file.
If there are no changes in the model state after assimilation and a visual examination of obs_seq.final was not informative, convert the obs_seq.final file to netCDF with obs_seq_to_netcdf and either use the Matlab tools distributed with DART or something of your own. Actually, obs_seq_to_netcdf works on all observation sequence files, not just obs_seq.final files.