When you are in the shop, look at the drink you know is free and check the calorie content. Compare it with the one you want to buy. If it is the same, or near enough, then it will also be free.

You can also check the calorie value of anything from Tesco on their website. There is 1 calorie per 250ml in their cherry cola which makes it about 8 calories for a 2 litre bottle. There are about 10 calories in a 2 litre bottle of Diet Coke, which is free.