<p>In the Render method of an ASP.NET web-control, I need to alter the output of the Html based on whether JavaScript is enabled or disabled on the clients browser,</p>

<p>Does anyone know the right incantation to figure that out?</p>

The
noscript
tag is used to define an alternate content (text) if a script is NOT executed.
EDIT: Hi Kieron, take a look at this link:
Creating a Server Control for JavaScript Testing
, and
Detect if JavaScript is enabled in ASPX