How can one detect via Javascript if a print stylesheet is in effect?

function getActiveStyleSheet()
{
    var i, a;

    for (i = 0; (a = document.getElementsByTagName("link")[i]); i++)
    {
        if (a.getAttribute("rel").indexOf("style") != -1
         && a.getAttribute("title")
         && !a.disabled)
            return a.getAttribute("title");
    }

    return null;
}