I'm also wondering the same thing. Running CPU-Z returns MediaTek MT6592, 8x ARM Cortex-A7 @ 1.66 GHz.
I've looked at some other videos and discussions online on how to tell a fake one apart, but for the most part it looks real, has waterproofing seals, a 16MP camera, heart rate monitor (though i have my doubts as to its results), etc. The Android version is 4.4.2. The fingerprint scanner doesn't seem legit, though, as i don't really need to slide my finger over the home button and any finger will unlock it. All in all i'm not really bummed as it was a third of the price of the other S5's listed online and still is much better than my previous phone. Really i just wish i could find a way to root it... I actually never really considered that it might be fake, though i was a little curious after reading about some of the things here:
ibtimes.co.uk/samsung-galaxy-s5-10-amazing-hidden-features-you-never-knew-about-1445367
...notably the touch sensitivity, which i don't have as a setting.
The battery life is also really weird, when opening certain apps (QQ, Anki) the battery life rapidly falls (completely gone within a couple hours) and i've got to restart the phone for it to stabilize again. I can watch movies or listen to music for what seems like days, though.