The same way I found out about the other food allergies: the hard way. I tried the papaya green tea at Panera. I drank the whole thing and then thought, "Um, why is my tongue itchy and why am I having asthma symptoms?" I'm fairly certain it was the palaya as the other thing in it was passion fruit, and I know for a fact that's fine. Took some benadryl and claritin. Feeling tired and a little loopy, but better.
Now the list is pineapple, strawberry, nectarines, tangerines, guava, peanuts, tree nuts, pine nuts. And a shrimp sensitivity and a milk sensitivity (I only do cheese regularly, and milk products like ice cream on a limited basis).