is there a way to check for a specific index on which dashboards this index is used?
Give a try? - Replace "your index name" accordingly in your search
| union
[| rest splunk_server="local" "/servicesNS/-/-/data/ui/views"
| search "eai:data"="*index=<your index name>*"
| eval Type="Dashboards"
| table Type title eai:acl.app author eai:acl.perms.read eai:acl.perms.write]
[| rest splunk_server="local" "/servicesNS/-/-/saved/eventtypes"
| search search="*index=<your index name>*"
| eval Type="Eventtypes"
| table Type title eai:acl.app author eai:acl.perms.read eai:acl.perms.write]
[| rest splunk_server="local" "/servicesNS/-/-/data/ui/panels"
| search "eai:data"="*index=<your index name>*"
| eval Type="Pre Built Panels"
| table Type title eai:acl.app author eai:acl.perms.read eai:acl.perms.write]
[| rest splunk_server="local" "/servicesNS/-/-/admin/macros"
| search definition="*index=<your index name>*"
| eval Type="Macros"
| table Type title eai:acl.app author eai:acl.perms.read eai:acl.perms.write]
[| rest splunk_server="local" "/servicesNS/-/-/data/models"
| search "eai:data"="*index=<your index name>*"
| eval Type="Data Models"
| table Type title eai:acl.app author eai:acl.perms.read eai:acl.perms.write]
[| rest splunk_server="local" "/servicesNS/-/-/saved/searches"
| search search="*index=<your index name>*"
| eval Type="Saved Searches/Alerts/Reports"
| table Type title eai:acl.app author eai:acl.perms.read eai:acl.perms.write]
But I would not go with this approach. It didn't work for few indexes for me though. You have to verify each dashboard if your index is referenced.
didn't work