Knowledge articles are attached to incidents via the m2m_kb_task many to many table, so if there IS a KB article attached to an incident there will be a record in that table with references to both the article and the attachment. If you want a list of incidents that don't have an article attached you could write a background script that queries the incident table, iterates through each incident, and queries the m2m table to see if there's a corresponding record.
The other option, if you needed to run a an actual report, would be to create a database view with a left join on incident that joins incident and m2m_kb_task together. From there you could look for incidents without a kb.
Unfortunately, there's not an easy oob way to get that data and each approach will require some work.
