Are you asking about when the tape is being written to? or when you are seeking on the tape to find the end?
I don't think you can do that when it is writing, but you can do it while querying a tape.
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There is a command "mt" that manipulates the tape. It has many functions. You can set marks on the tape and many other things as well.
From the man page for mt.
mt accepts the following commands:
eof ...... Write count EOF marks.
fsf ...... Forward space count files.
fsr ...... Forward space count records.
bsf ...... Backward space count files.
bsr ...... Backward space count records.
rew ...... Rewind tape.
offl ...... Rewind tape and go offline.
eod ...... Seek to end of data (DDS and QIC drives only).
smk ...... Write count setmarks (DDS drives only).
fss ...... Forward space count setmarks (DDS drives only).
bss ...... Backward space count setmarks (DDS drives only).
status ...... Print status information about the tape drive.
res ...... Reserve tape drive for sole use by the host issuing the mt command (stape driver only).
rel ...... Release tape drive from sole use by the host issuing
the mt command (stape driver only).