The arithmetic, which is the whole argument
Several of the world's most famous teas come from very small defined areas. The core Longjing production zone around West Lake is a few hundred hectares. The Wuyi zhengyan — "true cliff" — area is small and bounded. The original Anxi Tie Guan Yin heartland is a district, not a province. Darjeeling's entire annual output is a tiny fraction of India's. The volume sold worldwide under each of those names is very much larger than those areas yield. That is not an allegation about any particular seller; it is a statement about two numbers that do not reconcile, and it holds however honest the individual transactions are.
Four different things get sold under a famous name
THE PROTECTED ARTICLE. Grown in the defined area, to the defined method, frequently with a certification mark. Small volume, high price, and usually sold with paperwork. THE SAME STYLE FROM NEARBY. Made by the same method, from the same cultivars, in an adjacent district, by people who have made it for generations. This is most of what is sold, it is legitimate tea, and it is often good. The name is being used as a style descriptor. THE STYLE FROM SOMEWHERE ELSE ENTIRELY. Longjing-style green tea from a different province; "Darjeeling-style" from Nepal. Again frequently good, and the further the material travels from the name the more the name is doing work it should not. THE ACTUAL FRAUD. Material misrepresented deliberately, sometimes with forged certification. It exists and it is the smallest of the four categories by volume. The useful skill is not detecting fraud. It is knowing that a famous name most often means the second category, and pricing your expectations accordingly.
Where the name is legally defended
TeaHQ's own reading of well-attested general knowledge, with no single source behind it.
Some names have enforceable protection and some do not, and the difference matters. DARJEELING is a registered geographical indication with a certification mark, protected in India and in a number of other jurisdictions, and it has been litigated internationally. A packet using the word without authorisation is doing something legally actionable in those jurisdictions. Several Chinese teas hold national geographical-indication protection domestically, with defined production areas and standards. Most tea names have no such protection anywhere. "Ceylon", "Assam", "English Breakfast", "Earl Grey", "jasmine tea" and "gunpowder" are categories, styles or trade descriptions rather than protected origins, and "Earl Grey" in particular is a flavouring specification with no origin content at all. So the question "is this genuine" is only answerable for names where a definition exists to be genuine against.
What actually protects a buyer
PRICE ARITHMETIC FIRST. Fine tea from a small famous area is labour-intensive, hand-picked, seasonal and scarce. A price far below what that implies is telling you something before any other evidence does. This is the single most reliable signal available and it requires no expertise. SPECIFICITY SECOND. A seller with real material can usually tell you the sub-district, the harvest window, the cultivar and the maker. A seller with the style rather than the origin usually gives you the famous name, an adjective and nothing else. Ask; the answer is informative either way. SEASON THIRD. Fine spring Chinese greens are a spring product. A vendor offering the same mingqian tea all year at the same price is selling something whose relationship to the season is worth asking about. CERTIFICATION LAST, because it is the easiest thing to imitate and the hardest for a buyer to verify.
The reframe worth taking away
"Is this real Longjing" is usually the wrong question and it makes a buyer anxious about the wrong thing. Better questions: is this good tea, is the price consistent with what it claims to be, and does the seller describe it specifically or only impressively. A very good Longjing-style green tea from outside the core zone, honestly priced, is a better purchase than a mediocre one with an unverifiable certificate. The name is a proxy for quality and it is a poor one; the things it is a proxy FOR can usually be asked about directly.