Ask a Teacher

It's hard to say definitively how things worked for ancient people, because of the lack of records. But here are some possibilities.

Trial and Error: We would test things we'd eat before gathering enough to feed our tribe. If we had adverse reactions, we'd move on.

Observation: Watch other animals and see what they're eating, then go and try to get some ourselves. Not always going to work, but if it didn't kill them, it might not kill us.

Imparting of knowledge: When the younger were taught how to forage, they were shown by the elder (who had already discovered some) what is and is not safe to eat, and how to harvest it.

Please remember this. While ancient peoples did not have the wealth of knowledge we did, we should not assume they were less intelligent because of it. They had the same ability to deduce and to analyse as we do in modern times, but a different skill set due to different environment. They probably found ways to test foods before eating them that reduced the risk of death.