You must differ between ADTF 2.x and ADTF 3.x and their (adtf)dat file structure.
ADTF 2.x:
You are right, you can only interpret data with ddl. The stream must point to a structure described in Media Description.
Sometimes the .description files are incomplete or are missing link
from stream name to corresponding structure.
You can avoid this by enable the Option Create Media Description in Harddisk Recorder. Then a *.dat.description will be stored next to the same-titled *.dat file, which contains the correct stream and structure reference, because it was available during recording.
Is there some additional information about structure name hidden in the .dat file itself?
No, it is only the stream name. So you need to know the data structure behind to interpret. If you have the header (c-struct), you can also convert to ddl and refer to that.
ADTF 3.x:
To avoid these problems for not available or incorrect description files, the DDL is now stored in the *.adtfdat file in ADTF 3.x