I think once pure honey has been heated to a certain temperature, it will never crystallize, while pure raw honey will always crystallize after some time. I don't know if this is reliable for telling them apart, or how hot they have to be heated to create enough damage to prevent crystallization from ever occurring. And I'm sure soon they will come up with some chemical that makes heated honey crystallize anyway.
In the meantime, and until I find a better way to tell them apart, I will only buy honey that is already crystallized.
As a personal experience, a friend and I used to buy honey from a shady supplier who was selling what she claimed was organic honey, and it would always come already crystallized. Once in the summer, after a week or two of temperatures ranging from 35C to 40C (95 to 104F) in the city (likely lower in the countryside, but we didn't ask where it was coming from, so it may have been somewhere where temperatures got a bit higher than that), we ordered four 1 kilo jars and they came in liquid form. When asked about it, the seller said it was just because they had harvested it that week and that it would crystallize quickly if we put it in the fridge. That was 6 months ago, and the two I got never crystallized. My friend ate his anyway since he's not that into raw and he was having it with tea anyway, but he agreed that the taste was different and worse. I think that maybe they were exposed to the sun during or after extraction during those hot days, and that that may have heated the honey to the point of no return. Or maybe just temperatures as low as those 40C (104F) in the shade are enough to destroy it's ability to crystallize. Or maybe it was a coincidence, and while this honey was heated more during extraction, it had nothing to do with the weather and the temperatures required for this to occur are much higher.
