There is no way to tell if you have already played an island except by looking at the favorites, which is only helpful if its an island you actually liked.
This is horrible for many games but specifically horror games and story games that are meant to be played a single time as people will queue up to play a game, spend several minutes loading into it, and then leave it as soon as they remembered that they already played and beat the game and it negatively impacts both players and creators.