WooCommerce custom place order validation

I would like to do som additional checks when the user has submitted an order in Woocommerce. Have tried using different add_filter methods, but it seems that I can't hook up to a place_order filter. I have tried with:

add_filter('woocommerce_place_order', 'checkFields'); //on ordersubmit
function checkFields(){
  if($this != true){
    //not allowed to place order because of data is not true
  }else{
   //continue order
  }
}

But non of above is being triggered when I submit the checkout form. Any suggestions? :)

try something like this..

add_action('woocommerce_after_checkout_validation', 'rei_after_checkout_validation');

function rei_after_checkout_validation( $posted ) {

    // do all your logics here...
    // adding wc_add_notice with a second parameter of "error" will stop the form...
    // wc_add_notice( __( "OMG! You're not human!", 'woocommerce' ), 'error' );

    if (empty($_POST['captcha'])) {
         wc_add_notice( __( "Captcha is empty!", 'woocommerce' ), 'error' );
    }

}

$posted is an array like this..

Array
(
    [terms] => 0
    [createaccount] => 0
    [payment_method] => cheque
    [shipping_method] => 
    [ship_to_different_address] => 
    [billing_first_name] => 
    [billing_last_name] => 
    [billing_company] => 
    [billing_email] => iamhuman@gmail.com
    [billing_phone] => 
    [billing_country] => PH
    [billing_address_1] => 
    [billing_address_2] => 
    [billing_city] => 
    [billing_state] => 
    [billing_postcode] => 
    [order_comments] => 
)