When a dashboard shows a number, it also makes an implicit claim: this value is current, comparable, and safe to use. That claim needs evidence.
My preferred approach is to make source health part of the user experience. Show when each source was collected, whether its definition matches the displayed metric, and what the interface does when data becomes stale.
This changes the dashboard from a collection of charts into a decision system. The result may look slightly less magical, but it becomes much more useful when the underlying world is messy.