title: Javascript: How to tell whether AJAX response is JSON
body: <p>I've got an AJAX request that expects JSON in response.</p>
  
  <p>But there's a possibility that what gets returns may not be JSON, but rather an HTML error page (unfortunately, with response type 200).</p>
  
  <p>How can I tell whether the response is JSON or not?</p>
  
  <p>(I'm using jQuery, if that helps.  But I can't use any plugins.)</p>