I know this is an older question, but I felt compelled to chime in. If you goal is to simply determine if a site uses JSON for data exchange, then the solution proposed by Tim is a very good solution. However, if you are looking to scrape data from an arbitrary site, there is no guarantee that the site uses JSON as data exchange, as stated by @KenanZahirovic. There are numerous sites that do not do this. Instead they embed the data into the HTML, or use XML, or some other method for getting the content to the client. There is no standard way of doing it, which is why data scraping is so difficult. It requires figuring out how data is transferred and building a client for that.

For scenarios where you need to gather data from multiple sources, you may end up with multiple clients due to the nuances between the sources. This site explains some best practices for data scraping. However, this would likely require a server-side application. Having a server-side application that gathers data and stores it in a database would make the most sense for this scenario. This way you can have a consistent API that the client hits to access the data.

An algorithm has been proposed that can scrape many sites fairly confidently. If you only want to have a client, this may or may not be the best way to go. It all depends on how much processing you want the client to do. If at all possible, try and off load processing to a server.

This answer might be way more than is required, but ,again, I felt compelled to chime in. I am sure the previous answer was sufficient. I do recommend though that you mark an answer as the accepted answer.

Best of luck!

You can do this with JSONP like this:

function insertReply(content) {
    document.getElementById('output').innerHTML = content;
}

// create script element
var script = document.createElement('script');
// assing src with callback name
script.src = 'http://url.to.json?callback=insertReply';
// insert script to document and load content
document.body.appendChild(script);

But source must be aware that you want it to call function passed as callback parameter to it.

With google API it would look like this:

function insertReply(content) {
    document.getElementById('output').innerHTML = content;
}

// create script element
var script = document.createElement('script');
// assing src with callback name
script.src = 'https://www.googleapis.com/freebase/v1/text/en/bob_dylan?callback=insertReply';
// insert script to document and load content
document.body.appendChild(script);

Check how data looks like when you pass callback to google api: https://www.googleapis.com/freebase/v1/text/en/bob_dylan?callback=insertReply

Here is quite good explanation of JSONP: http://en.wikipedia.org/wiki/JSONP

If you want to do it in plain javascript, you can define a function like this:

var getJSON = function(url, callback) {
    var xhr = new XMLHttpRequest();
    xhr.open('GET', url, true);
    xhr.responseType = 'json';
    xhr.onload = function() {
      var status = xhr.status;
      if (status === 200) {
        callback(null, xhr.response);
      } else {
        callback(status, xhr.response);
      }
    };
    xhr.send();
};

And use it like this:

getJSON('http://query.yahooapis.com/v1/public/yql?q=select%20%2a%20from%20yahoo.finance.quotes%20WHERE%20symbol%3D%27WRC%27&format=json&diagnostics=true&env=store://datatables.org/alltableswithkeys&callback',
function(err, data) {
  if (err !== null) {
    alert('Something went wrong: ' + err);
  } else {
    alert('Your query count: ' + data.query.count);
  }
});

Note that data is an object, so you can access its attributes without having to parse it.

If you look in Chrome inspector, you probably see this error:

XMLHttpRequest cannot load http://webapp.armadealo.com/home.json. Origin http://stackoverflow.com is not allowed by Access-Control-Allow-Origin.

What this means is that the server doesn't want the client web page reading the file. The client isn't trusted. This is a basic security feature of XMLHttpRequest in order to prevent a site like mybank.evil.com from downloading data from mybank.com. It unfortunately makes testing from a local file challenging.

If you trust any site with your data or a select number of sites, you can configure your server script to send the Access-Control-Allow-Origin to allow certain sites through.

See https://developer.mozilla.org/en/http_access_control for more details.