title: jQuery Validation plugin: how to check if an element is valid?
body: <p>A little bit of context:</p>
  
  <p>I'm using the <em>jQuery Validation plugin</em> to validate a sign-up form. I now want to implement an <strong>ajax call to check whether the user name is available</strong> in the system, and I want to make this ajax call only if the userName value is a valid one as per the rules set in <code>$(form).validate();</code></p>
  
  <p>I want something like:</p>
  
  <pre><code>$("#userName").keyup(function () {
      if ($("#userName").isValid()) {
          //make ajax called
      }
  });
  </code></pre>
  
  <p>I searched the documentation but i couldn't identify the solution to my problem.</p>