How to determine if a font is a fallback font?

[Urs Liska]

Hi all,

#(ly:font-config-get-font-file "Emmentaler-13")

gives me the full path to the font file used by the "Emmentaler-13" 
font. If I use a non-existent font name, say
#(ly:font-config-get-font-file "SomeFont")
I get a reference to emmentaler-11.otf, which is obviously used as 
fallback font by fontconfig.
However, a Windows user reported that the same references "marlett.ttf" 
on his system.

I wanted to use this approach to determine if a requested font exists on 
the user's system, but now I'm not sure anymore if that's a reliable 
approach. Of course I can simply add the check for marlett.ttf, but I 
have the impression that the returned fallback font isn't hard-coded but 
can vary with the user's installation or font availability.

So:
1)
Does anyone know of another approach to determine if a given font is 
visible to fontconfig?
(I can't check for the file name because this doesn't necessarily match 
the font name)

or
2)
Is there a function to retrieve the "font name" from a given file name?
It would be a viable approach if I could compare the result of that 
request with the original font name.

TIA
Urs