First, judge from the color and shape of the tea leaves. The color of genuine Longjing Tea is yellowish-green and it is strip-shaped and flat. If the color is aquamarine, there is no doubt that it is fake West Lake dragon well green tea loose leaf.
Second, judge from fragrance. Authentic organic West Lake Dragon well Tea has a peculiar fragrance (known as the fragrance of beans), which is unique to Longjing Tea.
Third, judge from the color of the tea-water. After brewed with water, the color of genuine dragon well green tea-water is close to the color of brown rice. It is clear and bright rather than aquamarine.
Fourth, judge from the taste. Dragon well GreenTea has an unique mellow sweet and aftertaste. Generally, the tea-leaves picked before the Tomb-Sweeping Festival is quite tender, so that the taste will become mild after brewed two or three times.
Made from leaves that have not been oxidized.
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These are all very valid characteristics of genuine longjing, but these days other nearby regions can produce very similar tea. I can imagine, it's still very hard for the average drinker to separate real from fake. In the end, I would just worry about whether you like the tea you purchased or not.