I think you can't find that out, unless you - for example - write auditing triggers that'll write data you need into a table(s) you created.
DDL statements, however, can be "traced" - issuing this query will show you last DDL statement against the requested object:
SELECT * FROM USER_OBJECTS WHERE OBJECT_NAME = '<your_object_name>';
I need know last update in data not in table structure, so I have to write trriger.
I'd suggest you add a last_update_date column to the table for the trigger to populate, rather than storing a single value for the whole table in a separate table - otherwise your users' updates will serialize as they take turns to maintain the value!