Install this as a bookmarklet and it will show the site's colophon if it uses TypeKit.

bookmarklet.js

javascript:(function(){s=document.createElement('script');s.type='text/javascript';s.src='https://gist.github.com/raw/701397/typekit-colophon.js';document.getElementsByTagName('head').item(0).appendChild(s);void('')})();

typekit-colophon.js

(function() {
	var a = document.getElementsByTagName('script'),
		i, l, result,
		rurl = /^http:\/\/use\.typekit\.com\/([0-9A-Za-z]+)\.js/;
		
	for (i = 0, l = a.length - 1; i < l; i++) {
		if (result = rurl.exec(a[i].getAttribute('src'))) {
			window.location.assign('http://typekit.com/colophons/' + result[1]);
			break;
		}
	}
	
	if (!result) {
		alert('This site doesn\'t use Typekit!');
	}
})();

This code is based on markpasc's and pdokas' gists, which is a great improvement over mine. I combined the two for exponential awesomeness.

Fails when a kit is served from use.typekit.net instead of use.typekit.com. Time for an update?