The following is a bookmarklet I just created to check alt attributes on a web page. Tested on Firefox 8.1.

javascript:var%20imgs%20=%20document.getElementsByTagName('img');for%20(i=0;i<imgs.length;i++)%7Bif%20(imgs.item(i).hasAttribute('alt'))%7Bif%20(imgs.item(i).getAttribute('alt')=='')%7Bimgs.item(i).setAttribute('title','***%20empty%20alt%20attribute%20***');%7Delse%7Bimgs.item(i).setAttribute('title',imgs.item(i).getAttribute('alt'));%7D%7Delse%7Bimgs.item(i).setAttribute('title','***%20no%20alt%20attribute%20***');%7D%7D//released under GPL 3.0

To use, click on the bookmarklet in the bookmarks toolbar, then hover over each image.

Hope this helps.