Better question: how do I ensure that the mystery powder I bought is actually what I asked for? There's a good, user-friendly testing service in Vancouver. Do your homework, and your due diligence, or you can die. I wish this was an exaggeration.
I meant no criticism, just feel obliged to psa that. “do your homework" means learning about how and how much and what to expect. Due diligence is making sure you got what you paid for before you're having difficulty breathing
I did some very cursory research on these a few weeks back, and I ultimately decided against buying anything atm, at least in the clearnet sites. While many of the "research chemicals" are legal, sounds like a handful people have ended up in jail for quite a while, even for legal substances.
Yeah even different places within the US will probably differ. I just happen to live in a very conservative area atm with a particularly infamous police force.