I think is not an problem in your WP installation. The site works as encoding UTF-8, also your feed etc.
Validating
For an validating of strings should use the PHP function mb_check_encoding. A small script should check your database tables, the content so that you have feedback about your data inside the database tables.
Also it the libraray tchwork-grekas/utf8 helpful, for finding problems also fix the wrong strings.
Fix them via custom script
However if you will only search in your database, search via a plugin for an strings should helps you. Alternative is an custom script, that check and fix them. I think the forceutf8 library is helpful. The method fixUTF8 fix your problems, if you have inside your data.
Alternative is also the library Patchwork-UTF8 in the first abstract, see above.
mySQL Table Collation
Before you should check the Collation of all tables. You check their encoding by looking at the Collation value in the output of SHOW TABLE STATUS (in phpMyAdmin or Adminer this is shown in the list of tables).
SHOW TABLE STATUS FROM <YOUR_DATABASE>
mySQL Variable
You can also check each variable, run the follow mysql command in your database to verify that everything has properly been set to use the UTF-8 encoding.
SHOW VARIABLES LIKE 'char%';
Convert
Convert the table to InnoDB and utf8mb4, the posts table
ALTER TABLE wp_posts ENGINE=InnoDB ROW_FORMAT=DYNAMIC;
ALTER TABLE wp_posts CONVERT TO CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci;
WP Config
Check the wp-config.php
utf8mb4 is your choice since WordPress 4.2, full UTF-8 support.
define( 'DB_CHARSET', 'utf8mb4' );
Links
- Helpful post about switch from a NonUTF-8 database to UTF-8 for more details in the deep.
- Solving PHP MySQL UTF-8 issues