It's very difficult to find missing glyphs with a script. There's a hack: look for glyph ID 0 in all the document's used font, but this hack is unreliable as it doesn't work in overset text.

To test whether a single character is a missing in its applied font you can (try to) convert it to outlines. That too is an ugly hack, and can't be used for longer stretches of text.

Please add a property to the Story object, say, missingGlyphs, so that the call

myStory.missingGlyphs

returns an array of glyphs missing in a story.

Thank you.