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>