check-userjs.user.js

// ==UserScript==
// @name      check-userjs
// @namespace http://www.hatena.ne.jp/hitode909
// @include   http://*
// @include   https://*
// ==/UserScript==

var anchors = document.querySelectorAll('a[href$=".js"]');

for (var i = 0, len = anchors.length; i < len; i++) {
    if (anchors[i].href.match(/\.user(?:-\d+)?\.js$/)) {
        check(anchors[i]);
    }
}

function check(anchor) {
    var cache = GM_getValue(anchor.href);
    if (cache == "true") {
        anchor.href += '#';
    }
    if (cache) {
        return;
    }
    GM_xmlhttpRequest({
	method  : "GET",
	url     : anchor.href,
	onload  : function (req) {
            if (req.responseHeaders.match(/^Content-Type: .*html.*$/m)) {
                GM_setValue(anchor.href, "true");
                anchor.href += '#';
            } else {
                GM_setValue(anchor.href, "false");
            }
        }
    });
}