There is one liar card. Liar cards are inverse their values (so if it's a 2, it counts as -2). Try to find and pick the liar card each round. The number on top of cards is the sum of the numbers (so if the cards are 1, 5, 2 and the sum is -2 than the liar card must be 5, because 1+2 - 5 is -2).
Made while waiting for PGMA round 1 to finish being judged Idea by yours truly >:D