title: Determine if an object property is ko.observable
body: <p>I'm using <a href="http://knockoutjs.com/" rel="noreferrer">KnockoutJS</a> version 2.0.0</p>
  
  <p>If I'm looping through all properties of an object, how can I test whether each property is a <code>ko.observable</code>?  Here's what I've tried so far:</p>
  
  <pre><code>    var vm = {
          prop: ko.observable(''),
          arr: ko.observableArray([]),
          func: ko.computed(function(){
              return this.prop + " computed";
          }, vm)
      };
  
      for (var key in vm) {
          console.log(key, 
              vm[key].constructor === ko.observable, 
              vm[key] instanceof ko.observable);
      }
  </code></pre>
  
  <p>But so far everything is false.</p>