Determining whether a glyph is available in Tkinter

[wmcbrine at gmail.com]

I have a Tkinter app that can optionally label some buttons with certain Unicode glyphs that aren't always available (depending on the OS, etc.). When they aren't available, Tkinter renders them as "\uNNNN". What I'd like to do is check whether the glyphs are available, and fall back to my own alternate text for the button if not. Can I do this?

I'd also like to do the same in pygtk.