How to check in js that user has checked the checkbox in Google recaptcha?

I have added the following before end of head

<script src='http://ift.tt/1xQsaAf'></script>

I have added this before end of form

<div class="g-recaptcha" data-sitekey="== xxxxxx =="></div>

I can see the recaptcha similar to http://ift.tt/OgRQlq

HOwever, when user presses data without checking the checkbox, the data is submitted. Is there any other code I need to add to check if user has pressed the checkbox? Hopefully in js?