In my project, I had to troubleshoot a dashboard that was running as the previous admin and needed to find other dashboards that needed updating. Below are some queries I used in Workbench or Dev Console to identify the dashboards that required updating, sharing it for future use.
Query for all Dashboards running as Inactive users
SELECT Description,DeveloperName,Id,RunningUserId,RunningUser.isactive, RunningUSer.name, runninguser.profileid, runninguser.profile.name FROM Dashboard WHERE RunningUser.isactive = false
Query for all Dashboards running as a specific user
SELECT Description,DeveloperName,Id,RunningUserId,RunningUser.isactive, RunningUSer.name, runninguser.profileid, runninguser.profile.name FROM Dashboard WHERE RunningUserid = ‘userid_value’
Stay safe, stay home!!