I thought I could automate this buy printing a square with A, B, C, D on the back of each page, but as mentioned, A is not always at the top. It doesn't really take all that long to write the values on the back and if it is done lightly, it should impede cheating. I think the best option is to keep them down until you answer or hold them on your chest/face as others have mentioned.

If you simply print them, put them back in the printer and print again on the back side, the left/right will be flipped so you will have to print a mirror image and then the text is backwards.

For little kids (Kdg or beginning first grade and/or special needs kids who do not know letters or read yet,) my colleague, Kim, developed a "back" to paste onto the cards. Each side of the card has a colored square on it. So where the "A" is on the front, if you check the back, there is a red square. Where there is a "B" on the front there is a yellow square and so on. So when a child wants to choose "A" for a response, he makes sure he is looking at the back of the card and the red square is on the top. That way the front of the card is still facing me so I can scan it. Yes, cheating CAN be a problem, but we tell the students once they have chosen which color will be on the top, to hold the card against their chests (or in front of their faces) and then I scan it. It works well with the little kids. We're new to Plickers, so just trying to make it work for our littles. :)

I teach elementary kiddos, and once they decide on an answer I have them hold the card to their forehead. I have the letters written in the center of the back almost like a compass rose.

I teach 2nd graders and to make it easy to pass out card I lightly write names on back in pencil. I also have written the letters on the back edge lightly in pencil as well. They look at the back of the card and make sure the answer they choose is facing up. It didn't take me long to write them out. Pencil is written light enough others can't see but dark enough that the child can. Having the letters written on the back is a must for young ones!

I made small stickers to put on the back. Kids can see their letters to navigate, but classmates can't see.

I just wrote the letters on the back so the kids could see them.

I wrote the letters on the back of the cards before laminating. But found out today to my disappointment, that this enabled students to easily copy the answers of the students in front of them.
For that reason I won't be putting the letters on the back.

@Greg: this might be hard to make work - if we have the same pattern and rotation on the back, the answers won't match front-back: A-A, B-D, C-C, D-B.

So we'd need to have a different pattern on the back. This is something we're thinking about, but we might use it to add more answer choices.

Yeah, or just make the PDF easy to print each card double-sided so the kid's don't have to worry which way the face it.