Weapons against false-cards...
When declarer plays cards in a tricky order (really, is this a false-card), the weapon of choice are defense signals (count, suit preference, etc). Let's take Free's false-card example from the other thread (I show WEST hand)....
In Free's premise, you (south) know beyond a shadow of a doubt that the
♦K is with east. His idea was cash the
♦A and lead low, thinking EAST will duck. But here, EAST will have 1) gotten a count signal from WEST on Diamonds, and matter of fact, probably already has the 4 card ending counted out. So this "falsecard" is the only chance for an extra trick, EAST would have to either be asleep or have a partner who false=card him and gave him the wrong count somewhere.
But really, there is no official counter to a falsecard when they are done right. The best falsecard generally gives you an option of two plays where the lack of a falsecard would not. Dropping the NINE on the first round of a suit where you have J9xx to give declarer an option of ways to play for a 4-1 split is a case in point. IF you don't drop the nine, he will have to play the hand the suit so that the hand with the TEN would behind the four card suit without the nine being dropped.
The people to take advantage of is the ones who never false-card, or who always play the same card. Imagine this situation where a player holding QJ always plays QUEEN first when you cash the KING...
If they play the JACK, you are certain to win if you finessee because you know their behavior. Imagine a situation where you are in a trump suit with a singleton in dummy. IF when you lead the low card from dummy, and the RHO is known to always play the ACE in that situation, what do you play from a holding of KJx if he plays low? What if half the time he jumps up with ACE and half he plays low? These situations are similar to dealing with normal falsecards. If a player is known to falsecard, maybe you need to think about what has gone on before the play you suspect and if there is any evidence you have overlooked that might lead you strongly suspect the play is or is not a falsecard. But that is the best that the falsecarder can hope for, to give you a potential losing option and a nudge towards it.