navigate route with querystring

You need to add another route with that expecting parameter :

routes: {
    'posts?foo=:foo' : 'showPosts',
    'posts': 'showPosts'
},
showPosts: function (foo) {
    if(typeof foo != 'undefined'){
       // foo parameters was passed
    }
    test = true;
}

update
You could define the general route to return all the query string and then parse it in the handler :

routes: {
   'posts': 'showPosts',
   'posts?*queryString' : 'showPosts'
},
showPosts: function (queryString) {
    var params = parseQueryString(queryString);
    if(params.foo){
        // foo parameters was passed
    }
}  
...
// and the function that parses the query string can be something like : 
function parseQueryString(queryString){
    var params = {};
    if(queryString){
        _.each(
            _.map(decodeURI(queryString).split(/&/g),function(el,i){
                var aux = el.split('='), o = {};
                if(aux.length >= 1){
                    var val = undefined;
                    if(aux.length == 2)
                        val = aux[1];
                    o[aux[0]] = val;
                }
                return o;
            }),
            function(o){
                _.extend(params,o);
            }
        );
    }
    return params;
}

update 2

Here’s a live demo to see the code in action.