How did people determine what is edible safely back then?

heheh it didnt actual started with the slaves.thats bad example.actually it evolve for thousands of years of cooking meals.from hunting and gathering to sophisticated culinary.they usually taste it and based on experience tried to mix it with something else until they perfect it.

Long back our ancestors lived in jungles, so they ate this and that, fruits, roots, stem etc, those which were bitter, tasteless and poisonous were definitely were not eaten subsequently, it was certainly trial and error.. Professor Wishing you a merry X'MAS AND HAPPY NEW YEAR 2012 IN ADVANCE.

I think I heard that was the reasoning of the appendix

hmm, i think trial and error. one will eat it and see if they dont die.

I suppose just trial and error. I've certainly seen movies where medieval kings would have a taster eat their food to see if it was poison or inedible.

I think human beings figured out what was edible and what was not well before slaves were thought up. Even so, I think before using slaves, they would probably test food out on other animals first, and see what there reactions were. Nobody would want to risk losing a valuable slave. Also, there are edibility tests: you put the food on your skin, see if a negative reaction occurs in a while, if not, rub it on your lips, if good so far after with no tingling etc put it in your mouth, wait, if good eat a tiny bit, wait about 6-8 hours, if good, eat a large amount, wait, it good, eat as much as you want.