Currently, checking out user reviews is the only way to get a idea of how the vendors heroin is. I think the issue is, no one wants to send a sample of the dope they just bought from someone. So, here we are.
Unless there emerges someone that would buy small amounts from all vendors with at least some reputation on the markets and test it themselves... I think were out of luck.
No offense but heroin users aren't generally the most generous or trustworthy of people. I would not expect too much from the "steal your wallet but then help you look for it" crowd.