Is it possible to find when the last insert took place?
Not giving away any code or anything just yet but ways to detect whether a DB is still in use by some app can be as follows:
- turn the DB off (set to single user mode, set as read-only, etc) many variations to effectively get the result of someone shouting at everybody else because one of their processes stopped unexpectedly and its because of the server you just "turned off"
.. personally this is my favourite approach - but I do understand that it may cause some real business issues and some people just can't deal with emotions ;).


you could also monitor the database size and see if that changes
you could have a small process that counts all rows from particular tables (all tables) and logs that into some other database for you.. then simply see if the row count changes
you could monitor the log file (if anything happens in terms of INSERT, UPDATES, DELETES, then you will see that there.
you could run sp_who(2) at regular intervals to check who's connected


These are just quick and easy to implement methods to see if someone, somewhere is still using the database.

I hope this helps,

B