For example, consider pottery, and the invention of glazes. It's generally thought that the invention of the ore -> metal process involved potters experimenting with different glazes and finding lumps of metal after the firing process was complete.
With toxic plants, well, most peoples kept domesticated animals and would probably use them to test the toxicity of potential food sources. Feed it to the chickens, pigs, llamas, etc. and see what happens. Soak the potential food source in water, see what happens. Try adding other substances, like charcoal or clay, and see what happens.
Why don't people recognize this? Simple - infantilization of native peoples as a justification for the ravages of European colonialism.