1//You can call the function getCookie with the name of the cookie you want, then check to see if it is = null.
2
3function getCookie(name) {
4    var dc = document.cookie;
5    var prefix = name + "=";
6    var begin = dc.indexOf("; " + prefix);
7    if (begin == -1) {
8        begin = dc.indexOf(prefix);
9        if (begin != 0) return null;
10    }
11    else
12    {
13        begin += 2;
14        var end = document.cookie.indexOf(";", begin);
15        if (end == -1) {
16        end = dc.length;
17        }
18    }
19    // because unescape has been deprecated, replaced with decodeURI
20    //return unescape(dc.substring(begin + prefix.length, end));
21    return decodeURI(dc.substring(begin + prefix.length, end));
22} 
23
24function doSomething() {
25    var myCookie = getCookie("MyCookie");
26
27    if (myCookie == null) {
28        // do cookie doesn't exist stuff;
29    }
30    else {
31        // do cookie exists stuff
32    }
33}