It'r pretty easy for me. If it gives me boils, gas, bloating and pain - then I KNOW it's bad. If science tells us it's not food (refined oils, synthetic anything) then I avoid it. If it's not something I'd feed to my own baby, I shouldn't eat it. Anything GMO like ALL American corn, wheat and soy - is instantly off the table.
That said, at least once every two weeks I eat something deplorable. Sometimes I suffer for it, sometimes I don't. But every time I eat vegetable oil or trans fat, I KNOW I just mummified my arteries a little, and they're probably permanently hardened a little more every time. Debatable? Yeah. Good way to stay away from poison? Guaranteed.
Steak, eggs, potatoes - fruits, nuts, berries and forage. Coconut milk and potent herbs and spices. Tea instead of coffee now and teeny amounts of kelp daily. Let's see how this does! Not really had dairy much, and gut seems better for it.