How can I see $post object in frontend from functions.php?

Adding:

global $post;
print_r ($post);

alone, won’t work in functions.php (becuase WP’s core functions etc wouldn’t be loaded). It would inside a relevant template file, but not in functions.php.

To make it work in functions.php, you need to do it inside an function that is hooked to an appropriate hook. The “appropriate” hook may depend on what $post object you’re looking for (e.g. that of a single post or a post type archive), but a good way to target many situations is the wp_head hook, e.g:

add_action('wp_head', function(){
    global $post;
    print_r ($post);
});