Use the below query to query all private reports in Salesforce.

Sample SOQL:

SELECT Id, Name, Owner.Name FROM Report USING SCOPE allPrivate

To know more about USING SCOPE, check the below link

https://developer.salesforce.com/docs/atlas.en-us.soql_sosl.meta/soql_sosl/sforce_api_calls_soql_select_using_scope.htm