My mother has it several years now, and she can't eat any read meat whatsoever. I had never heard of it before and neither and she. She was diagnosed by an allergist through bloodwork.
They look for the alfagal proteins.
It's beginning to appear that there are other ways to develop this allergy other than a tick bite (as the allergy is starting to show up in a few people outside of the normal range), but tick bites (from a particular species of tick) appear to be the primary cause.