The only way I know to test is to use a DTS WAV file. If you play it in Foobar and it's bit-perfect, the DTS decoder (receiver) will see it as DTS instead of PCM and play it accordingly. Any resampling or attenuation destroys the DTS signal and will cause the file to be played back as noise. Preferably you would use a 44.1khz file, so you could be sure that CD or FLAC isn't being altered. (Some sound cards will resample 44.1khz but not 48khz, so that DTS on DVDs will pass.)
Of course, you'll need a receiver with DTS to do this, and make sure the volume is turned down very low because if it plays noise, it could damage your speakers. And, obviously make sure that any processing in the playback software is turned off. There are some files here:
Free 5.1 Surround Sound DTS and AC-3 Downloads You'll have to unzip them to get to the WAVs.