[XeTeX] Detect, whether a font contains a certain character

[Tobias Schoel]

Hi,

I know, this question has been raised several times on this list. But 
I'm bad at searching, so I'm just asking:

How can I detect, whether a font contains a certain character (let's say 
u2103=Degree Celsius symbol) from within XeTeX?

Application:
If a font has this glyph, I would like siunitx to use it as the unit 
symbol for \celsius.
If a font doesn't have this glyph, I would like siunitx to use °C as the 
unit symbol for \celsius.

Thanks

Toscho
-- 
Tobias Schoel
Europaschule Kairo
www.europaschulekairo.com