gistfile1.js

// feature test for position fixed.

Modernizr.addTest('positionfixed', function () {
    var test    = document.createElement('div'),
        control = test.cloneNode(false),
        fake = false,
        root = document.body || (function () {
            fake = true;
            return document.documentElement.appendChild(document.createElement('body'));
        }());

    var oldCssText = root.style.cssText;
    root.style.cssText = 'padding:0;margin:0';
    test.style.cssText = 'position:fixed;top:42px'; 
    root.appendChild(test);
    root.appendChild(control);
    
    var ret = test.offsetTop !== control.offsetTop;

    root.removeChild(test);
    root.removeChild(control);
    root.style.cssText = oldCssText;
    
    if (fake) {
        document.documentElement.removeChild(root);
    }

    return ret;
});

jQuery actually has some detection code for this as well:

https://github.com/jquery/jquery/blob/eed380/src/offset.js#L125-143

Current state of this detect is here Modernizr/Modernizr#167