title: Check if object is an Observable
body: <p>I am using RxJS 5 and have this method:</p>
  
  <pre><code>Queue.prototype.drain = function (obs, opts) {};
  </code></pre>
  
  <p>in the method, I would like to check if the user passed in an <code>Observable</code> for the first argument or if they omitted the <code>Observable</code> and just passed in an options object.</p>
  
  <p>So basically I need to do something like this:</p>
  
  <pre><code>if(!Rx.Observable.isObservable(obs)){  //this method is fictitious
      opts = obs || {};
      obs = Rx.Observable.interval(1000);
  }
  </code></pre>
  
  <p>I assume RxJS provides users with this kind of check but I cannot find the documentation that shows you how to do this type checking.</p>
  
  <p>Anyone know how?</p>