In Diablo 3 our hero will state that a certain person is Belial.
Emperor Hakkar is Belial, and be correct.
How does the hero know?
It's the plot question rather than the game itself.
Also, if possible, I'd like the "official" reason.
For example, the idea that Belial can teleport is a speculation, not official reason why we know that person is Belial.
Best Answer
Common Sense.
You get ambushed by demons in his throne rooms, he teleports everywhere you go and talks about stuff which wouldn concern him if he would have been just a kid emperor.
Also for a kid, he seems to be pretty wise. How does he know about Zoltan Khuul? How does he get in his lair before you open it up ? I mean come on, he is like 8 years old.
It is really really obvious. Atleast Blizzard noticed that, and assume the Player Character (Even Barbarians with low intelligence) will see behind this masterful plan by the Lord of Lies.