I think the easiest answer is by testing it. I don't think a purity and substance test isn't that expensive.

You roll the dice, having learned the odds are in your favour.

"The primary sources of street Ketamine in China include diversion or theft of legal pharmaceuticals from medical or veterinary licit trade with some supplies also manufactured in clandestine laboratories (United Nations Office on Drugs and Crime, 2010). It is therefore believed that the purity and the quality of street ketamine is high (no research data available) contributing to its increasing popularity among drug users in China." (2014)

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4004368/

In any system where an actor can be either honest or dishonest but certain qualities of correctness can be probabilistically ascertained, consensus is often a great replacement for trust.

In the case of an anonymous decentralized marketplace, vendor attributes like product quality can be ascertained by achieving consensus in the form of ratings and reviews.

Ultimately, without running the product through a mass spectrometer yourself, you are going to have to defer trust, so consensus aims to reduce the likelihood of dishonesty over an average.

For people in some parts of the world, the level of trustworthiness gauged from online marketplaces can supersede the level of trust given to local vendors.

Take Amazon for example, which, while not anonymous, uses consensus to assure quality in the form of validated reviews. Many people already prefer shopping by reviews than by trial-and-error at brick and mortar stores.

How do you trust anything funded by companies who desire slanted research? You trade one governing body for another. In the darknet's case, you lean on consensus and ratings.