Of course this excludes the case where the contributor has identified h[er,im] self.
When you click on "Edit" in the life sketch it will show who was the last person that edited the sketch. It will only show the person's contact name. (The name they have chosen to show publicly in Family Search).
After viewing (editing) the Life Sketch the contact name seems to be shown without having to edit the Life Sketch. Is this the behavior you see/expect?
As well as being able to see who made the most recent change when you click on Edit on the Life Sketch, you will see who contributed the Life Sketch in Latest Changes and you will also see any other changes made by others to the sketch. In fact, there is a filtering mechanism in Latest Changes that allows you to filter on Life Sketch.
Here is the knowledge article that explains Latest Changes:
Even better answer: Changes to Life Sketches are included in the Latest Changes Show All report. Use the browser search to find "sketches" in that report.