After playing for a few turns you start getting a sense of who has which cards, especially if the other players start taking them off you by attacking you! It also pays to watch who seems to be clearly heading for a particular island…
Example: At one point my daughter and I were racing to an island to drop off a King or Queen there in return for the Ace. Unfortunately my son managed to get there first with a Jack: he ransacked the island!
This trumped both cards: as you cannot replace a lower card with a higher one, once the Jack is exchanged on the island no further exchanges are possible.
Therefore a Jack is possibly the most powerful face card in the game. It’s worth the least points, but it can always be exchanged on an island for something else, and it can stop any other higher card taking treasure.