Identify when Table Schemas were last Modified
When InnoDB add an index it adds a record to I_S.INNODB_SYS_INDEXES. INDEX_ID of a new index will be highest. Analyzing existing records you can get an idea in what order indexes where created, but that's it. INNODB_SYS_INDEXES doesn't contain any time fields.

On other hand, any DML will be logged to a binlog. If it's enabled and it hasn't been rotated yet you will see the ALTER TABLE with a timestamp.