Extremely hidden. I have done it on my Bicycle decks and Tally-Ho decks and no one has seen it. They are all very subtle signs. Once you get used to reading the cards, you can read them in a second.
In card games, I can win every single time. My friends and I played poker once (just with chips, not for real money) and I did not get caught at all. In fact, I won too easily because when I dealt, I just remembered what cards everyone recieved, and when I didn't deal, I watched closely to try and catch the marks as they were being dealt.
I was not as successful when I did not deal but I still knew the cards of the two next to me.
Daniel Madison has about 5-9 tricks I think. Most of them are variations though. However, you will be able to think of others. Anything from a psychological reveal to the ends of your imagination.
The only thing I did not like about this was that it cost $20. I think that is a little high because although it is really clever, I think that if I was given a week to make a marking system for the Bicycle deck, I could have made one very similar.
Also, to make it you only need a sharpie that is the same color as your deck
