You know those empty blocks for characters when no suitable font is found?
Well Emacs 24 will show the hex code in the block. From C-h n (news):
"Enhanced support for characters with no glyphs in available fonts. If a character has no glyphs in any of the available fonts, Emacs normally displays it either as a hexadecimal code in a box or as a thin 1-pixel space. In addition to these two methods, Emacs can display these characters as empty box, as an acronym, or not display them at all. To change how these characters are displayed, customize the variable `glyphless-char-display-control'."