By doing an image comparison of the one you posted to the same image you posted on Facebook or Twitter, for example.
Or by doing a deeper comparison of markings between the image you posted and all the images indexed by Yandex, for another.
Yandex is actually pretty good at taking an image of someone in one pose, and finding the same person's images in other related poses.
If you ever want to know true fear, just go attract the attention of 4chan's /b/ or /pol/. Those guys located and called in an airstrike on a Syrian terrorist camp, found and stole Shia LeBoof's "HWNDU" flag three or four times when he got tired of being mocked and kept moving it, and have identified a bunch of criminals, animal abusers, and terrorists from little more than blurry partial-face photos, including Eric Clanton and Michael Reinoehl, through massive distributed look-at-every-photo-on-the-internet efforts.