This simple tool checks for the existence of a specified font. It ultilizes the font fallback mechanism in CSS for font checking.
Simply call checkfont, and it will return the font status in boolean:
var isTimesExist = checkfont("Times"), //true (if exists)
someWeirdFont = checkfont("CookieMonster"); //false
For whatever reason you need to test on a different DOM, another window for example (limited by SOP), you can pass that in too.
var win = window.open("/another_page");
checkfont("Century Gothic", win.document);