Playing cards can be marked by invisible ink. You can not see the marks by nake eyes, but when you wear our glasses, you can see the mark clearly. So we call this ink invisilbe. You can get this kind of invible ink and do the mark yourself.
A disturbing trend I've noticed in the poker community is that people seem to want to follow strict guidelines when they play poker. Many want to treat poker like blackjack, where there is an optimal strategy in every situation.
This is simply not the case. Any and all advices are just some general hints that can never take the place of proper situational judgment skills. Poker is a game of people and a game of situations. It is not a game of optimal, pre-planned strategy. All of the best poker players vary their play and make decisions on the spot. Poker players that rely on a ready-made recipe are doomed to fail because they will play very predictably, and they won't take into account many important situational factors when making decisions.
There is really no way to teach someone situational judgment skills. The only advice on this topic that I can give is to practice and pay attention to your flaws. What are some situations that could be played differently? How often do you correctly place opponents on their hands?
Something that may be helpful is online poker hand histories. Some, but not all, poker rooms will display the hands of all the losing players who called a showdown (the screen will show mucks but the hand history will show the opponents' hands). Some poker rooms that do this are Party Poker and Titan Poker. When you beat someone who called you to the river, request the hand history and see what your opponent had. This will help give you an idea of how often you correctly judge your opponents.
If you are going to play poker, have confidence in yourself. Think for yourself. Don't worry if your play may be violating textbook guidelines. A winning poker player's arsenal combines general poker knowledge with situational judgment skills. Losing poker players don't think for themselves or simply don't think at all.
The invisible ink is not very suit for mahjong, as it is hard to dry and easy to be wiped. Although we have this kind of ink, we'd also suggust you that you'd better send the poker here to process, as we are professional and the card looked more beautiful than you do.