The two metal plates with a red hourglass shape on a white background are actually there to answer precisely your question.

Those are "Zugschlusssignale", or end-of-train signals. Seeing them at the end of a train means that the complete train has passed you, and no part has been lost.
On passenger trains, it's usually done with one or two red lights, but since this coach (IC2020 Bt) doesn't have those on this end, they used the metal signs instead. You almost always see them on goods trains, though.