I'm going through this at the moment, and my best solution so far is to open up every DTS package, view the package properties (on the toolbar or right click on any white-space in the package), go to the "Logging" tab, and then choose to enable logging to your local database server. Click OK, and save the package.
Then, you can go to MSDB and look at the package history:
SELECT * FROM msdb.dbo.sysdtspackagelog
If anyone has a better idea, I'd love to hear it, but this seems like the best way of knowing exactly when each package is run.