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

Google has a call back option for when the checkbox is checked.

Add this to your form element:

data-callback="XXX"

Example:

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

And a disable attribute to your submit button.

Example:

<button id="submitBtn" disabled>Submit</button>

Then a create a callback function and write whatever code you need.

Example:

function recaptchaCallback() {
    $('#submitBtn').removeAttr('disabled');
};