The easiest way to find out is to load the firmware and see if it works.
As with anything that comes in versions, some work, some don't. I haven't tried to track down why. So far, I have not found any "genuine Arduinos" that don't work.
I have an Arduino that came from Inventables and both the J Tech version and my version work on it. My Arduino has the socketed CPU chip. Some of the Inventables versions have the SMD chip.
I also have several clones --- they work too.