how do you determine end of tape? (floppy)

[Jay A. Snyder]

the tar that comes with SCO xenix knows when I gets to the end of a
floppy, and then prompts for another.  Does anyone out there know how
this is done?  I want to write a program that I can pipe the output of
a program through so that I can write multiple floppies worth of data
to foppies.

The reason I want to do this is:

tar zcvf - | floppyswapprogram > /dev/fd0

J