This should work too. If the JavaScript on the browser is disabled it will output a message and a link on how to enable it.
<noscript style="font-size: 18px; color: red; font-weight:bold">Javascript is disabled:<a href="https://www.google.com/adsense/support/bin/answer.py?hl=en&answer=12654"target="_blank">Click here to learn how to enable it before continue.</a> <br/></noscript>