From https://serverfault.com/questions/340464/how-to-list-the-files-on-a-tape-with-tar
To see what format a backup is in you can use something like this:
dd if=/dev/rmt/0m of=/tmp/tapehead bs=1024 count=2 file /tmp/tapehead
Means: copy the first 2k (2 blocks of 1024 bytes) from the tape to /tmp/tapehead
and then run the file command on it to see what format it is.
This assumes that your /etc/magic
file contains the given format description.
If the file command cannot identify a specific file type, or just returns 'data', you might be able to identify the format using a cat, more or a text editor combined with Google. Some backup programs (Amanda for example) even include a text file as the first file on the tape describing how to extract the tape.
See Troubleshooting guide if the dd command results in an I/O or memory allocation error.