[NTG-context] \goto with references which don't exist, how to spot?

Hello, I use something like \goto{chapter}[some-reference] where this chapter is not yet written (and thus I have later to check whether it is indeed called 'some-reference' and not by chance 'some-other-reference'). Unfortunally, ConTeXt writes 'chapter' and not e.g. chapter~{\bf ??} or chapter~$\blacksquare$ or something else which is easily spotable. (I like the square, a '\bf ??' is too easily be overlooked.). How can I make ConTeXt show such a sign if it cannot resolve a reference? Tobias