Syntax:
SHOW INDEX FROM tbl_name [FROM db_name]

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;

SHOW KEYS is a synonym for SHOW INDEX. You can also list a table's
indexes with the mysqlshow -k db_name tbl_name command.

URL: http://dev.mysql.com/doc/refman/5.0/en/show-index.html

