You can use opaque sleeves, or a "checklist" card that is included in the promo slot of packs that may contain transforming cards. These checklist cards have a normal Magic back.
711.3. Players must ensure that double-faced cards in hidden zones are indistinguishable from other cards in the same zone. To do this, the
owner of a double-faced card may use completely opaque card sleeves or
substitute a checklist card (see rule 713).
- Checklist Cards
713.1. A checklist card is a game supplement that can be used to represent a double-faced card or meld card.
713.2. A checklist card has a normal Magic card back. The face of a checklist card is divided into sections. Each section lists the name
and mana cost of each double-faced card or meld card it could
represent and includes a fill-in circle. Before a checklist card can
be used, exactly one of the fill-in circles must be marked to denote
which card the checklist card represents.
713.3. If a checklist card is used in a deck, the card it represents is set aside prior to the beginning of the game (see rule 103.1a) and
must remain available throughout the game. A checklist card can’t be
included in a deck unless it is representing a double-faced card or a
713.4. For all game purposes, the checklist card is considered to be the card it’s representing.
713.5. If the checklist card is face up in a public zone, it should be set aside and the double-faced card or meld card that it represents
should be used instead.