Smell it or brew a little to see if there's a moldy odor. Generally, Pu-erh tea has an aged aroma, which is different from mold. The four key principles for selecting Pu-erh tea are "clear, pure, proper, and aromatic":
First, clearly smell its aroma. Regardless of the Pu-erh tea's raw/ripe type, age, quality, shape, or price, the first step is to smell the tea. After fermentation and aging, Pu-erh tea will certainly have an aged flavor, but it should not have a moldy smell. A moldy odor indicates that the storage space was damp or too humid, lacking ventilation. So-called aged but not moldy: the aged flavor will dissipate with the steam when brewing, while moldiness comes from tea deterioration due to internal and external dampness and mold growth.
Second, purely distinguish its color. Before brewing, smell whether it has a clean aroma (no strange or foul odors), then brew it. When Pu-erh tea is stored in normal conditions, even after 30, 50, or even 100 years, the tea color (tea soup) will definitely not turn black or develop strange smells.
Third, store it properly. Pu-erh should be stored in a clean, ventilated environment.
Fourth, tea easily absorbs surrounding odors. The brewed tea's aroma can help judge the aging environment and oxidation time.
The Pu-erh tea market is not yet fully standardized, and there are few merchants capable of authenticating Pu-erh. Driven by profit, some falsely label teas as vintage or aged, with 20, 30, or even 50-60 year old rare Pu-erh becoming increasingly common. Many merchants store tea in dark, highly humid environments to accelerate aging, even artificially sprinkling water, often causing tea leaves to mold and deteriorate. After rapid spoilage, they dry the tea cakes, quickly producing "wet storage tea" that resembles aged Pu-erh. If it smells mold-free and the brewed tea soup is reddish-brown without strange odors, then you can confidently taste it. What looks like mold might be your misconception, as Pu-erh tea buds and leaves are very robust with dense fuzz, so processed tea has white hairs that might initially resemble mold. Pu-erh tea nature is complex; drink cautiously, and avoid heavily moldy tea.
It's said that if it's white mold, it might be okay. Pu-erh tea concentrate has its unique aging cycle, theoretically about 60 years. Beyond this period, after internal substance conversion completes, along with crystal water deterioration, quality declines. The concentrate may weather or mold. Thus, don't easily believe market claims of high-year "old tea concentrate"; even if real, nutritional components are likely depleted, lacking drinking value. Note that in recent years, some Pu-erh concentrate products have appeared among civilians, even some so-called old concentrates, mostly made using modern "imitation" methods like pot boiling, quite different from imperial tea room techniques. Tests on several alleged old tea concentrates in recent years detected modern pesticide residues, proving they used contemporary tea leaves and differed greatly from imperial production methods.