> Of course, if I buy rat poison, I have a pretty pure product and I can look up the ld50 and make a scientific estimate on how much I can take before I die. Not so with some random shit off the street.

True.

Where I live (Aotearoa) drug testing has been made legal and there are many opportunities to take your stash along to the spectrometer (I am not a chemist, unsure about the actual nature of the instrument, except it is reliable) and get it tested.