I don't suppose there's any configuration option or something that would cause fonts to only load when the text would otherwise be not displayed properly?
Topic on Extension talk:WebFonts
It is not possible to detect that programmatically.
It's actually impossible to check whether some characters are supported? Couldn't you just check if every letter in a character set results in the same offsetWidth as a box character and each other, or maybe write the text to a canvas and check the result, or something?
That sounds.... not very reliable. Eek!