In the case of honey, it's actually very simple: if it's viscous outside of summer, it's not honey. Natural honey crystalizes into (near-)solid form quite quickly.
Has it been treated in any way? I've been working with bees for over 20 years now, and I haven't seen honey keep the viscosity for more than a couple of weeks past harvesting, at room temperature. The duration varies based on the blossoms of course, with some such as rape going solid in 10 days while clover can last 4-6 weeks, but in the end, it always crystalizes.
That's true, but how "quickly" depends on the kind of honey. Acacia honey and other honeys with high amounts of fructose and low amounts of glucose can take decades to crystallize, normal forest honey or dandelion honey can crystallize in under a day.