How do I check if an index exists on a table field in MySQL?

Use SHOW INDEX like so:

SHOW INDEX FROM [tablename]

Docs: https://dev.mysql.com/doc/refman/5.0/en/show-index.html

Try:

SELECT * FROM information_schema.statistics 
  WHERE table_schema = [DATABASE NAME] 
    AND table_name = [TABLE NAME] AND column_name = [COLUMN NAME]

It will tell you if there is an index of any kind on a certain column without the need to know the name given to the index. It will also work in a stored procedure (as opposed to show index)

SHOW KEYS FROM  tablename WHERE Key_name='unique key name'

you can find if there exists an unique key in the table