How do you guess the final card in the game Set?

There's a card game called Set, and when you're playing and all of the sets have been correctly filled, then how do you guess, from the final batch, what the last card is?|||Are we playing the same game?





Set as my family plays it ends when the last three cards are dealt into the twelve already in play and then one or more sets are claimed until no more sets can be found.





Most times the game ends with twelve cards still in play left unclaimed. Where do you get "the final card"?