Well, you wouldn't know because both tails might be below or above the line due to the kurtosis. In those cases, both the cases below the line or above the line could be ether left or right skewed. To say this another way, a normal Q-Q plot can be used to examine data that is not normally distributed, and in so doing, the skewness is only indirectly shown. One method of determining skewness is to just calculate it.
Positive skewness has been said to have a longer or fatter right tail. Fat and long tails are not quite the same things.