His jewels? Though there are lots of them in this painting and they suggest that this man is very rich, they do not tell us that Henry was a king.
It's not his hat either, because any wealthy Tudor man may have worn his hat.
The correct answer is nothing. There are no real clues to tell us that the man in the painting was a king. Royal portraits usually had symbols to show who the person was and how powerful they were, e.g. crowns. Holbein did not use such symbols in this portrait. Henry is wearing a special garter (the Order of the Garter) but this only tells us that he was high-ranking nobility or a prince.