Somewhere on the label it will give you the calorie content. There is no free food allowance for drinks, so you can use the 1 syn per 20 calories guideline.
So, if it says that it is 80 calories per 100ml (as an example, I have no idea what it really is) then it will be 4 syns per 100ml.
I have a feeling that it might turn out to be rather high in sugar, but the only way you will know is by reading the label.