There are chances that you want to check for the indexes that you created on the tables in past because there can be a large number of indexes on numerous of tables depending on the size of the database
If you want to see the indexes for a particular table
SHOW INDEXES FROM <table_name>
And when you have to see all the indexes for the complete database you have to use this command
WHERE TABLE_SCHEMA = 'your_database_name';
Removing the where clause will show you all indexes in all databases.