How to find the unclosed div tag

A unclosed div problem almost make me crazy. It is very difficult to track especially when the page is long and complex.

<div>
   <span>
       <b>Text</b>
       <a href="/">Title <span>another test</span>
   </span>
</div>

How can I find unclosed HTML-tags on a website?

Any suggestions?

Solution

As stated already, running your code through the W3C Validator is great but if your page is complex, you still may not know exactly where to find the open div.

I like using tabs to indent my code. It keeps it visually organized so that these issues are easier to find, children, siblings, parents, etc... they'll appear more obvious.

EDIT: Also, I'll use a few HTML comments to mark closing tags in the complex areas. I keep these to a minimum for neatness.

<body>

    <div>
        Main Content

        <div>
            Div #1 content

            <div>
               Child of div #1

               <div>
                   Child of child of div #1
               </div><!--// close of child of child of div #1 //-->
            </div><!--// close of child of div #1 //-->
        </div><!--// close of div #1 //-->

        <div>
            Div #2 content
        </div>

        <div>
            Div #3 content
        </div>

    </div><!--// close of Main Content div //-->

</body>