How to detect if an other front-end post/page is in HTTPS/SSL from admin to display a notice?

You could get all posts/pages, check if the shortcode for your plugin is used, and if so retrieve the permalink for that post/page.

Other than that, I don’t know of any reliable method. And while I think your intentions are good, this does not really belong in your plugin, it isn’t really your job to educate people.