ACF does not pass this information. Post ID will always have an idea and will not be ‘new_post’ when acf/save_post is called.
If you are using a front end form I would suggest using acf/pre_save_post.
If this is an admin form then the only way to do this would be to use the built in wp save_post hook with a priority > 10 https://codex.wordpress.org/Plugin_API/Action_Reference/save_post
