Event.implement({

   hasCapsLock: function(){
      return ((this.code > 64 && this.code < 91 && !this.shift)
           || (this.code > 96 && this.code < 123 && this.shift));
   }

});

And then access the method from a keypress event:

$('test').addEvent('keypress', function(event){
   if (event.hasCapsLock()){
      // do something
   }
});

The only drawback is that it relies on sniffing alphabet characters and whether the shift key was pressed. This means that if the user presses the caps lock key, you won't know it until another character is inserted.

First, include this snippet where you want the suggestions to appear. For WordPress blogs, this is typically the 404.php file in your themes folder ((The full path would be /wp-content/themes/yourtheme/404.php. Create it if it doesn’t exist)).

Optionally, edit its look and feel by extending this base stylesheet. Tip: you can hide, for example, the search box, by adding display: none to #goog-wm li.search-goog

    /* Widget content container */
   #goog-wm { }

    /* Heading for "Closest match" */
   #goog-wm h3.closest-match { }

    /* "Closest match" link */
   #goog-wm h3.closest-match a { }

    /* Heading for "Other things" */
   #goog-wm h3.other-things { }

    /* "Other things" list item */
   #goog-wm ul li { }

    /* Site search box */
   #goog-wm li.search-goog { }

    /* Search text input */
   #goog-wm-qt {}

    /* Search button */
   #goog-wm-sb { }

Then you’re done! Check mine out (deliberately wrong url) for an example.