> dd if=/dev/sd0 of=/dev/null count=1
> fstat /dev/sd0

Or more simply

  </dev/sd0 fstat /dev/sd0

The dd approach will send the writer a SIGPIPE as soon as dd exits, so you
might not catch it with the fstat.  This way the writer will be conveniently
blocked while you look, because it's trying to write and fstat isn't reading.

-Chap