How can I tell if an index is being used to sort in MySQL?
You should append EXPLAIN EXTENDED before query and see result yourself.

It should have an entry for 


possible_keys


If this column is NULL, there are no relevant indexes. In this case, you may be able to improve the performance of your query by examining the WHERE clause to check whether it refers to some column or columns that would be suitable for indexing.

and 


Keys


The key column indicates the key (index) that MySQL actually decided to use. If MySQL decides to use one of the possible_keys indexes to look up rows, that index is listed as the key value.

For more information you can refer this Explain output and Explain join_types