...
- Go to the site in question.
- Go to /mod/simplecertificate/verify.php
- For example, if the certificate of completion was issued on by My Dual Code, which is located at https://my.dualcode.com, you would go to https://my.dualcode.com/mod/simplecertificate/verify.php
- Manually enter the code in the text area
- Click on the "Verify Certificate" button
...