Using Outlook Express you can save.eml files into a drop folder to make sure a policy will generate incidents as expected.
Within the Symantec DLP Enforce UI (Administration -> Overview) you will notice that the processed messages increased by 1.
If an incident was created from the test email, it will have a timestamp from when the email was actually saved. You can sort the incident list by incident ID. Since the test email was one of the latest ones to trigger an incident, it should show up with one of the higher incident ID numbers.