I used this to determine which actions were firing, and their order:
add_action( 'shutdown', function(){
foreach( $GLOBALS['wp_actions'] as $action => $count )
printf( '%s (%d) <br/>' . PHP_EOL, $action, $count );
});
And that showed me that the ninja forms plugin uses a custom action nf_display_enqueue_scripts
, which is what I used.