How do people magically get their hands on these various substances and know that they’re authentic?
It costs about $100 and a couple weeks but you’ll get anonymous spectral analysis of your molecules.
As for authenticity, my impression is that a huge proportion, if not most, of the exotic drug trade is mis-identified and/or mis-dosed. As others have pointed out precautions exist for the careful and meticulous. But let's be realistic, this does not describe 90% of recreational drug users.
As for this substance I've only had what was described as a "child dose" and enjoyed the absolute tranquility, some friends have had bad trips on much higher doses but as someone that isn't a fan of psychedelics and didn't want any hallucinations I loved that small dose.
Some people sell psychedelics in person. In my country they're incredibly rare and I've never seen anyone sell anything except mushrooms or LSD. So the fallback is the darknet.
>and know that they’re authentic?
If you're buying from the darknet, user reviews. Other people make good guinnea pigs. Otherwise, there are reagents you can use to test to make sure you've got DMT. But generally people just smoke it and see what happens. DMT is very unique, if the substance you have is something else it will be noticeable very quickly.