Once the sauce has cooked and is still hot, you can very carefully taste the cranberry sauce to check how tart it is. Remember when tasting it that it should be quite tart and not sweet. The tartness is what balances the meal and cuts through the richness of the holiday food. If you think it is too tart for you though, while it's still hot, add some more sugar to taste. This should be done while it is hot because the sugar will dissolve easily