Materialized View: How to determine whether original tables were changed?

Try:

SELECT * FROM all_tab_modifications

Adjust the query to fit your needs.

<edit>
I just realized that it might not be the latest modification of the table you're asking for, but the latest modification of the tables content.
In that case you can run this query instead:

SELECT  scn_to_timestamp(max(ora_rowscn)) 
FROM    Mytable

</edit>