how to difference whether the wine?

[Innkeeper]

Now I don't know whether you are talking about actual sweetness (residual or added sugar after fermentation) or perceived sweetness (from the fruitiness in the wine). Neither Merlot, nor Shiraz, nor Cabernet Sauvignon are normally fermented sweet or off dry. Some of any of them might taste sweet because sugar has been left or added (one method is adding unfermented grape juice), or has an abundance of fruit flavor. We drink more Shiraz and Cabernet Sauvignon than Merlot because they usually have more dimensions than Merlot. That does not mean to say we don't enjoy an exceptional Merlot from time to time.