See that rectangular part, soldered at either end, just below the USB connector? Brown between the two terminals? What's the marking in the middle there?
That's the PTC fuse, and at least for a while, Arduino was using ones with custom marking on them so people could tell the difference between official and counterfeits.
On almost all clones and counterfeits, the PTC fuse is green (a counterfeit is marked to look like a real one, with the Arduino trademark and "made in italy" even when it's made in china, whereas a clone doesn't have the logo on it and doesn't hide the fact that it's not official).
That said, why do people care so much about whether the boards are real? The clones, even the counterfeits, work fine. I have literally dozens of pro mini and nano clones, and only 4 real boards total, all of which were gifts.
Some clones require different drivers (CH340G instead of 16u2 - that board definitely uses 16u2), but clones that don't work are rare (except from WAVGAT - their boards are notorious for compatibility problems)