Opponent hand cards are denoted with icons

• - coin
• - mulliganed
• - returned (e.g. by youthful brewmaster)
• - created cards (e.g. spare parts)

• Everything in the overlay can be moved around by unlocking it in the options menu:

• What's displayed in the deck list can be moved around and turned on/off separately, too:
  (same goes for the opponent, one item lower in the list on the left)