Quality of what, exactly
The word covers at least four separate things: whether the leaf is what it claims to be, whether it was made carefully, whether it is fresh, and whether you like it. Only the last is subjective and it is the one most often dismissed. A technically excellent tea you dislike is not a good tea for you, and a great deal of writing about quality exists to talk readers out of that entirely reasonable position.
What the dry leaf tells you
Look at uniformity first. Leaf that has been graded and handled carefully looks consistent in size and colour; a mixture of fragments, stalk and dust suggests either a low grade or a blend of several lots. Then smell it. Fresh, well-stored tea has a distinct aroma dry; if it smells of nothing, of cardboard, or of the cupboard, the tea has faded or absorbed something and no brewing technique recovers it.
What the wet leaf tells you, and why nobody looks
The spent leaf is the most informative and least examined part of the process. Tip it out and look: whole unfurled leaves mean careful plucking and rolling; a mass of broken pieces means the opposite. Colour tells you about oxidation and evenness — patchy colour means uneven processing. Tears and bruising that look mechanical rather than deliberate suggest machine handling. This costs nothing and tells you more than the dry leaf does.
What price does and does not track
Price tracks scarcity, labour and reputation, and only loosely tracks how much you will enjoy the cup. Hand-plucked spring tea from a famous mountain is expensive because of all three. A well-made everyday tea from an unfashionable region may be a fraction of the cost and better suited to how you actually drink. The strongest correlation with enjoyment is freshness, which is cheap, and correct brewing, which is free.
The one thing worth being sceptical about
Grading systems, competition awards and origin claims are all real and all faked. Where a claim carries a premium — a specific mountain, a specific year, a competition placing — the claim is worth exactly as much as the seller’s traceability. Absent that, read it as a description of intent rather than of fact, and judge the tea in the cup.