A. Using Workbench to call Analytics REST API
Documentation about this API - Salesforce Reports and Dashboards REST API.
2. Execute this service /services/data/v37.0/analytics/reports/00O3B000000FiSd
change 00O3B000000FiSd to your Report Id.
3. Click Show Raw Response link.
4. Scroll down and look for "reportMetadata".
From here, we can see the 1st Created By come from Account, while 2nd Created By come from Contact.
B. Extract Report Metadata
Use Force.com IDE or Mavensmate to open particular report metadata. Same as above, 1st Created By come from Account, while 2nd Created By come from Contact.
Reference: Salesforce Reports and Dashboards REST API