How to determine if an Index is uselss in Percona MySQL?
Percona themselves have written a tool for this very purpose - it's called pt-index-usage and is to be found here.

From the DESCRIPTION:


  This tool connects to a MySQL database server, reads through a query
  log, and uses EXPLAIN to ask MySQL how it will use each query. When it
  is finished, it prints out a report on indexes that the queries didn’t
  use.


Percona's servers, tools, blogs and employees are highly regarded parts of the MySQL ecosystem/community.