How do I know if an index has been successfully created in MySQL?
Welcome.

If you say that the script did not finish / exit...

One crude method to find if the indexing process is in the middle...

You shall try listing the folder contents of /var/lib/mysql/dbname/ and checking the date and time of update of the table names concerned. If the date/time does not change for a while you shall be sure that the process is not touching that table file for as much time. Hence cornering if there is a process snap due to timeout or other.