How do I find the fallback font of a unicode code point?

Question

I'd like to use DrawGlyphRun to draw multilingual text. How do I find the fallback font of the character which is not in the specified FontFamily?

if (glyphTypeface.CharacterToGlyphMap.TryGetValue(text[n], out glyphIndex)){
      //draw the character using glyphIndex
} else  {
       How do I find the fallback font of character text[n]?
}