sgml-fill-nobreak might helpfully have a docstring to tell you want its
nobreaking condition is.  I don't think it would show up in customize or
anything as such, but C-h v fill-nobreak-predicate in sgml-mode shows it
and you can follow it from there to see what it does.

2010-01-05  Kevin Ryde  <user42@zip.com.au>

	* textmodes/sgml-mode.el (sgml-fill-nobreak): Turn comment into
	docstring.