Testing arbitrary HTTP headers

[Cliff Wells]

I recently had an issue with Google Web Accelerator breaking one of my
AJAX apps and stumbled across this web page:

http://www.petefreitag.com/item/312.cfm

This seems a good idea for web applications, but I'm wondering how I can
achieve something equivalent to 

if ($x_moz = prefetch) {
    return 404;
}

since $x_moz isn't a predefined Nginx variable.  It also raises the
larger question of how to test against arbitrary HTTP headers.

Regards,
Cliff