"Mark linked collections" add-on help you to see in the outliner your linked collection by mark them, so you can know which collection is linked (from other file or from other scene).

I write this add-on after I worked on a project with a lot of scenes, and I have some collections that were linked from one scene to other scenes (like props or background collections). My problem was that I was not able to know which collection is linked, and sometimes I add animation or edit the objects in the linked collections and after some time I see that it was a linked collection, and it ruin my other scene with unwanted animations…

So now with this add-on you can know which collection are linked to another scene just by look at the outliner and avoid of ruin your projects by mistake.