I agree with DPsycho. I think this information should be visible, since the player can write such information to the game note, when opponent play reveal cards. However, I still wasn't able to find a good way to display it. Here are the options I tried:
1. Card border
Doesn't use additional space, but it's impossible to find a color that will fit all backgrounds. Letting the player choose his own background because of this feature seems very awkward.
2. Revealed card insignia
Put a small picture somewhere in the card to display that it has been revealed. This however uses additional space within the card and it doesn't look well. Only place where it wouldn't use the additional space would be the card picture. To make a double layer card picture would require not trivial modification. Also, the insignia picture would block part of the card picture.
3. Encoding this information into some other card content. For example card name can be displayed in italic style when revealed.
4. Put it somehere in newcard flag, maybe redesign newcard flags as well to save more space.
Any ideas where to put this information are welcome.