Validate Google reCaptcha in Magento 1.9

Please see the original Magento contact form controller, i.e. app/code/core/Mage/Contacts/controllers/IndexController.php -> public function postAction(). For example you have to add this in your sendemailAction function below $params = $this->getRequest()->getParams(); (don't forget to surround it with try/catch block):

        $error = false;

        if (!Zend_Validate::is(trim($params['name']) , 'NotEmpty')) {
            $error = true;
        }

        if (!Zend_Validate::is(trim($params['email']), 'EmailAddress')) {
            $error = true;
        }

        if ($error) {
            throw new Exception();
        }

This is PHP validation. If you like JavaScript validation - you can see again Magento original contact form for reference, i.e. app/design/frontend/base/default/template/contacts/form.phtml. So you have to add this at the bottom of your form:

<script type="text/javascript">
//<![CDATA[
    var simpleContactForm = new VarienForm('simple_contact_form', true);
//]]>
</script>

To do so, you need some custom code, please follow below steps which work for me:

Note: I assume you used jQuery library on your frontend theme

Step-1: On document ready add some custom class to your reCAPTCHA input field:

<script type="text/javascript">
jQuery(document).ready(function(){
    jQuery('#recaptcha_response_field').addClass('required-captcha-entry');
})
</script>

Step-2: Initialize Magento VarienForm javascript class object and add new validation rule for reCAPTCHA field:

<script type="text/javascript">
var yourFormValidationObj = new VarienForm('your_form_id');

Validation.add('required-captcha-entry', ' ', function(v) {
    return !Validation.get('IsEmpty').test(v);
})
</script>

Step-3: On form submit, validate form for required values

<script type="text/javascript">
if(yourFormValidationObj.validator.validate()){
    // Form validate and ready to submit
    jQuery('YOUR_FORM_IDENTIFIER').submit();
} 
</script>

That's, your reCAPTCHA field is become required field and validated before submit the form.