It's virtually always noted in the description, but puerh descriptions are notoriously self-promoting and unreliable. It seems to be normal business to sell a cake as pure
Nannuo (or
Yiwu or
Bulang or
Lao Ban Zhang or
Jing Mai, etc.) when there may be little or none of the advertised name in the actual cake or the material used may be only the lowest quality.
More often than not the vendor is as much in the dark as the consumer.
A tiny number of vendors, such as Nada and the famous owner of The Best Tea House in Hong Kong mentioned above by Tim, actually go to the location and oversee the production of the tea. This may or may not result in something you like. Both Jim at PuerhShop and Scott at Yunnan Sourcing have gotten involved to one extent or another in some "
house" puerh cakes that they offer through their stores.
This situation leaves the majority of us puerh novices in the unfortunate position of not really knowing what anything tastes like until we can get in some years of experience and have a bit of luck.
