"I think I have found a bug in Octave but I'm not sure. How do I know, and who should I tell?"
First see the section on bugs and bug reports in the Octave manual. The Octave manual is included in the Octave distribution.
When you report a bug make sure to describe the type of computer you are using the version of the operating system it is running, and the version of Octave that you are using. Also provide enough code so that the Octave maintainers can duplicate your bug.
If you have Octave working at all the easiest way to do this is to use
the Octave function bug_report. When you execute this function
Octave will prompt you for a subject and then invoke the editor on a
file that already contains all the configuration information. When you
exit the editor Octave will mail the bug report for you.
If for some reason you cannot use Octave's bug_report function
mail your bug report to "bug@octave.org". Your message needs to
include enough information to allow the maintainers of Octave to fix the
bug. Please read the section on bugs and bug reports in the Octave
manual for a list of things that should be included in every bug report.