9 track tapes and block sizes

[shadoooo]

Hello,
I have a question about 9 track tapes and block sizes.
What I know is that tape is subdivided in files by means of marks, and each
file is subdivided in blocks of equal size.
Programs like tar use a specific block size to create files on tape.
However files can have different block sizes like bootloader file,
installation dumps and root file system copy on 2.11BSD.
Now suppose you find and unknown tape you want to preserve: using dd you
could easily 1:1 copy tape files to hard disk files using a SCSI drive and
Linux.
But: how you know which block size is on the tape?

Thanks
Andrea