When you are doing reports clean-up, you may not be able to delete reports that are used as the data source for the dashboard.
So, we need to find out the report is used by which dashboard:
1. Create a new custom report type with primary object "Reports"
2. Report type label: "Reports with Dashboard information"
3. Store in Category "Administrative Reports"
4. Click relate to another object
5. Select "Dashboard Components"
Now create a new report using above report type created, I would like to add following fields in the report:
- Report Name
- Report Unique Name
- Report ID
- Dashboard: Title
- Dashboard Column
- Dashboard Component ID
Reference: How to identify reports attached to dashboards?