You'll probably want to monkey-patch the send method
app.use(function(req,res,next){
var send=res.send;
res.send = function(){
//use the arguments here to get your body out (won't always be the first property if there's a response code passed through)
console.log("Sending",arguments);
send.apply(res,arguments);
}
next();
})
look at the source for the response.send method to find out how to parse the arguments. https://github.com/visionmedia/express/blob/master/lib/response.js#L81