Syntax:
SHOW {INDEX | INDEXES | KEYS}
    {FROM | IN} tbl_name
    [{FROM | IN} db_name]
    [WHERE expr]

SHOW INDEX returns table index information. The format resembles that
of the SQLStatistics call in ODBC.
You can use db_name.tbl_name as an alternative to the tbl_name FROM
db_name syntax. These two statements are equivalent:

SHOW INDEX FROM mytable FROM mydb;
SHOW INDEX FROM mydb.mytable;

The WHERE clause can be given to select rows using more general
conditions, as discussed in
http://dev.mysql.com/doc/refman/5.1/en/extended-show.html.

You can also list a table's indexes with the mysqlshow -k db_name
tbl_name command.

URL: http://dev.mysql.com/doc/refman/5.1/en/show-index.html

