Are you an administrator or just a viewer and is this your own arcgis online site?
Depending on this, you can edit the dashboard and find the Web Map powering the dashboard (if it is a web map) and then inspect the data sources in that Web Map, or there may be Data Expressions powering some items which you wall have to inspect.
If you don't have these privileges, you could just open the developer tools (f12 for most) and inspect the GET requests sent when this data is interrogated and look for the source. This url source/service may point you to the likely owner.
