What functions are included in apply_filter(‘the_content’)

You can use the global $wp_filter array to check for callbacks on each filter. Here is for example a code snippet that prints out all the callbacks on the the_content filter in the footer part of your theme:

add_action('wp_footer',function(){
        global $wp_filter;
        printf('<pre>%s</pre>',print_r( $wp_filter['the_content'],true));
});

The priority 10 part could look like this:

   [10] => Array
        (
            [wptexturize] => Array
                (
                    [function] => wptexturize
                    [accepted_args] => 1
                )

            [convert_smilies] => Array
                (
                    [function] => convert_smilies
                    [accepted_args] => 1
                )

            [convert_chars] => Array
                (
                    [function] => convert_chars
                    [accepted_args] => 1
                )

            [wpautop] => Array
                (
                    [function] => wpautop
                    [accepted_args] => 1
                )

            [shortcode_unautop] => Array
                (
                    [function] => shortcode_unautop
                    [accepted_args] => 1
                )

            [prepend_attachment] => Array
                (
                    [function] => prepend_attachment
                    [accepted_args] => 1
                )

        )