First off, let me just say that you aren’t going to believe me until you try this experiment yourself, so I encourage you to try. Hold out someone’s left forearm face up and stick a piece of food on there that the person claims to be allergic to. Have them hold their thumb and ring finger together as tightly as possible. If they are indeed allergic, it will be very easy to pull their thumb and ring finger apart. If they aren’t, then it will be difficult.
We first tried this on Jenn, who has a major gluten allergy. When we put some white flour on her forearm, her fingers came apart like she wasn’t even trying. Then, we tried a blackberry, which she can eat fine, and her fingers could barely be pulled apart.
I then did this experiment on my mom, who can’t drink anything with caffeine in it. I tried with coffee beans, and her fingers came apart very easily. But when we tried with sour cream, it was very difficult to pull them apart.
Unfortunately, I have no allergies so I couldn’t verify that this worked on myself, but being witness to both Jenn and my mom made me a believer.