<script type="text/javascript">

	/**
	 * Actual function that does all the work. Returns an array with all the info.
	 * My Assumption is that most of the browsers will have arial set as their default sans-serif font.
	 */
	var Detector = function(){
		var h = document.getElementsByTagName("BODY")[0];
		var d = document.createElement("DIV");
		var s = document.createElement("SPAN");
		d.appendChild(s);
		d.style.fontFamily = "serif";		//font for the parent element DIV.
		s.style.fontFamily = "serif";		//have to use serif coz in FF3.0, it doesn't fall back to font of parent element.
		s.style.fontSize   = "72px";			//we test using 72px font size, we may use any size. I guess larger the better.
		s.innerHTML        = "mmmmmmmmmml";		//we use m or w because these two characters take up the maximum width. And we use a L so that the same matching fonts can get separated
		h.appendChild(d);
		var defaultWidth   = s.offsetWidth;		//now we have the defaultWidth
		var defaultHeight  = s.offsetHeight;	//and the defaultHeight, we compare other fonts with these.
		h.removeChild(d);
		/* test
		 * params:
		 * font - name of the font you wish to detect
		 * return:
		 * f[0] - Input font name.
		 * f[1] - Computed width.
		 * f[2] - Computed height.
		 * f[3] - Detected? (true/false).
		 */
        function debug(font) {
                h.appendChild(d);
                var f = [];
                f[0] = s.style.fontFamily = font;       // Name of the font
                f[1] = s.offsetWidth;                           // Width
                f[2] = s.offsetHeight;                          // Height
                h.removeChild(d);
                font = font.toLowerCase();
                if (font == "serif") {
                        f[3] = true;    // to set arial and sans-serif true
                } else {
                        f[3] = (f[1] != defaultWidth || f[2] != defaultHeight); // Detected?
                }
                return f;
        }
        function test(font){
                f = debug(font);
                return f[3];
        }
        this.detailedTest = debug;
      this.test = test;
	}
	var fonts = [];
	/**
	 * other stuff
	 */

var detective = new Detector();

if(detective.test('ここに判別したいフォント') == true){
}
else{
document.write('<link rel="stylesheet" href="hoge.css" type="text/css" />');
}

</script>