Jen, that 's a real tough question.
Food testing labs have equipment that can test the light-bending characteristics of honey as opposed to syrup. 'You and I can't quite do that trick.
If you have a real good good nose, you might be able to spot the absence of a floral fragrance. That is, providing you are not dealing with an intentional fraud, where perfumes can be added to the fake supposed honey.
Assuming your question is about a beekeeper who feeds syrup and doesn't want his/her real honey adulterated with syrup---then it just requires some advance planning. There are diferent ways to keep your honey pure---NEVER feed syrup when there iis a honey flow going on,
Remove syrup-containing frames from the hive when the honey starts coming in or make sure to mark the frames with syrup and keep them separate from the honey frames in the hive (preferrably in a different super).
I'm a firm believer in my method---I never feed syrup, only honey. When I melt down my cappings (if I don't use them for mead) I save the heated honey for feeding later on, should the hives run short of their own reserves. Heated honey is not as good a quality item as unheated, but at least it's the real thing.