Find source of Spotlight Dictionary result
The Dictionary app does tell you the source, though it is not intuitive. When you add a new &quot;source&quot; through Dictionary's Preferences, the Dictionary app adds a tab button for it on the top.

Example 1 - Oxford Dictionary:



Example 2 - Apple Dictionary:



When a search result is displayed in Dictionary, the tab button name highlights the source from which it found the result.

Sometimes, the tab button names are not helpful. For such cases, you can rename it to what you want:

Right-click the tab name, and click on Edit Label:



Type the new name, and click OK.



The tab button will now be renamed: