Moms with babies that only have blood in their stool

How are you eliminating foods you can't eat?

My baby doesn't always have blood in his stool, so I assume it takes a couple days to show up after I eat dairy or soy?

A doctor recently told me to also cut out soy, but soy is in everything especially at restaurants. I read soy oil is okay for some and not okay for others, but how would you know when the allergic reaction isn't immediately visible on the outside like a rash?

I want to eliminate items from a restaurant or a grocery store (like a rotisserie chicken from sprouts/costco).