only thing that was easy to answer was the title
beyond that i don't think i've ever read of an allergy diagnosed over the internet
beyond the 'have you checked "xxx", "yyyy", or zzz ???', it's kind of pointless.
- actually it's just as hard for a vet to properly diagnose the cause for an allergy in the real world and they are face to face with the dog
whether we think a dog is having an allergic reaction to food or the environment, all we usually do is treat the symptoms until the reaction resolves
or we spend a lot of time, (and often money) systematically and scientifically starting the process of "ruling out" what doesn't appear to be the cause until either we, or the vet, runs out of imagination
i had a friend who spent TWO AND A HALF YEARS doing this with their whippet.....made me sad to watch the process from the sidelines
....eventually gave up and attributed it to behavioral problems and a weak immune system caused by .... genetics