How do people know if you have allergies or not

how do people know they have allergies? some people have allergies to peanuts but how do they know it?

Allergies come in many different forms. Generally, we discover we have allergies when we have a reaction to something. The reaction may be a simple headache or rash, or something major, like not being able to breathe. If you are concerned that you may have allergies, see your doctor and get evaluated - having done the "unable to breath / swallow" type of allergy a couple of times, I promise you that you really want to avoid this experience.

Allergy to peanuts can be throat closing [sure sign you need to go to emergency NOW], or just an initial indication like mouth itches or tingles when you eat something. If by chance, you are not the possibly allergic to peanut person, please do not try to "prove" the allergy is all in that person's mind / imagination - the hassle of avoiding one or more foods due to allergy is just not any benefit we "drama queens" with food allergy are supposed to be getting...6 or 8 hours in emergency on a single visit will convince most people we aren't pretending...

food allergies cause a reaction immediately after ingesting the food. hives, rashes or more severe respiratory problems. the first time this occurs, most people seek medical attention and are tested for allergies to that food.

other (seasonal) allergies cause sinus and similar problems at various times of the year, depending on the allergy.

If they have a reaction towards the food

My grandma is allergic to crab meat and when she eats it her tongue swells.