Many wine tasters confuse the taste sensations of sweet and fruity, and those who DO get it, often find it difficult to explain the difference. So, how do you do it?
One way to tell the difference is practicing with a glass of tea. Taste a sip of tea on its own, and you'll find it to be bone dry, and perhaps even a bit tannic (that astringent sensation on your tongue). Now, add a squeeze of lemon to the tea and taste it again. The tea is still dry, but there is now a fruity (citrus) component from the lemon. Next add sugar or honey to the tea, and taste it a third time - you should find it to taste both fruity AND sweet.
If you want to try this experiment with wine, practice by tasting a Sauvignon Blanc next to a Riesling. The Sauvignon Blanc will offer tropical, citrus, "fruity" flavors, while the Riesling, which typically has more residual sugar (the amount of sugar remaining in wine after fermentation), will help you differentiate what "sweet" is.
Another way to determine if a wine is sweet, is by taking a look at the alcohol content. During the wine making process, the natural sugar in grape juice converts to alcohol. Higher alcohol wines typically produce a drier style because more of the sugar in the grapes is converted to alcohol. Low alcohol wines (under 12%) are usually sweeter.
Try some of these tests and see if you can tell the difference!