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

SHOW INDEX returns table index information. The format resembles that
of the SQLStatistics call in ODBC. This statement requires some
privilege for any column in the table.
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;

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

URL: http://dev.mysql.com/doc/refman/5.5/en/show-index.html

