>Is there a way to know the last datetime a certain table was updated? (Insert, delete or update) Other than writting triggers for each table, that is...

Not to the best of my knowledge.

I can see two solution. First, have a trigger that update's a timestamp in another table. The problem is that you've now introduced a hotspot. All transactions are trying to achieve a lock on this other table.
The second, is to store a last-update value on each row and update it with a trigger.

-Mike