How can I locate where the actions are defined? [duplicate]

You can find out what actions are assigned to given hook with this code:

function print_filters_for( $hook = '' ) {
    global $wp_filter;
    if( empty( $hook ) || ! array_key_exists( $hook, $wp_filter ) ) {
        return;
    }

    print '<pre>';
    print_r( $wp_filter[$hook] );
    print '</pre>';
}

Call it where you need it. In your case:

<form>
    <h1> My form <h1>
    <input type="text" />
    <?php do_action( 'woocommerce_checkout_after_customer_details' ); ?>
    print_filters_for( 'woocommerce_checkout_after_customer_details' );
</form>