A person will never know if they are allergic to a certain type of food unless they have eaten the food allergen itself. It takes a certain amount of food for the reaction to take place and trigger a symptom.
As you may have known already, food allergy is an unusual reaction to food that is elicited by an exposure to certain food types which then creates a response from the immune system. The reaction that the immune system eventually generates will then cause discomfiting symptoms.