MySQL Indexes: manage and determine index usefulness before remove. (Add comments)

One of the things I don't like about our webserver is there is no real way to see if an index is useful anymore.

I can easily see if tables or fields are used by searching the code of all the programs for the name of a field or a table.

When I run show indexes from Table I see a Comment column. Can I add notes to an existing index so that I can ref开发者_StackOverflowerence why the index was added?

Obviously it would be better if I could keep track of the frequency that a given index is used.

Any suggestions? (other then a separate commenting policy? I would rather allow comments to stay tied to the actual index which keeps them slightly more up to date and much more convenient)