Javascript is Huge now and since facebook and other large JS Based site, browsers have pushed out new innovations to deal with the latest web changes when it comes to JS
Because of this JavaScript is widely supported and you should not worry dramatically but it still needs to be delt with.
A Simple <noscript> tag to display a message to a user telling them they need javascript to continue
<noscript>
Please enable javascript to use this site
</noscript>
Keep checking your analytics software such as google, mint etc to see what your visitors are using.
That will help you decide how much you have to worry about it
A way to Track non JavaScript enabled browsers is like so
<noscript><img src="noscript.php" /></noscript>
This will load a php file as an image!
the php file
<?php
//Track the user details here inside your database or whatever so you can then see.
//Here send back a 1x1 pixal so the DOM IS ok and to reduce load
$pxl= imagecreate( 1, 1);
header( "Content-type: image/png" );
imagepng( $pxl);
imagedestroy( $pxl);
?>