[XeTeX] how to test whether a font exists?

[Grzegorz Murzynowski]

I know it's basic but I don't find in The XeTeX Companion a primitive 
for testing existence of a font.

It would be nice to write something like

\iffontexists "Garamond Premier Pro" \setmainfont{Garamond Premier Pro} 
\else \typeout{@@@@ no Garamond Premier Pro, leaving the default font}\fi

Any suggestions?

All the best,
Grzegorz Murzynowski.