1 answer
I've not tried to verify this, but two options that come to mind are:
The "out of the box" reports all have a create date of whenever your instance of Salesforce.com was created. You should able to filter on this date (i.e. reports created later than it).
Other option is that all the "out of the box" reports were created by the same person -- perhaps this person is no longer with your company, so you could filter on this as well.
