A Lying Riddle

You have before you two identical men. 

With just one difference and that's not noticeable externally: 
 - One ALWAYS lies. 
 - The other ALWAYS tells the truth. 

You don't know which tells the truth and which is the liar, but both of them know about that specific trait of the other. 

That is, the liar knows that the other is always truthful; and the truthful knows that the other is always a liar. 

You, I repeat, do not know how to distinguish them nor have you any means to do so. 

You have two balls, one white, the other black. 

You have two opaque boxes. 

The balls are put one in each box. 

Both men see in which box each ball is put in. You don't. 

You are entitled to ONE question, and one question alone, to any one of the men, to find out in which box the white ball is.  

What would that question be?

This is a known riddle. Not mine.

To find its solution you just have to follow the truth that a lie always says by being a lie, or better said, by being unavoidably the representation of its opposite: the truth.

Have you found what question that you are to ask?

Simple. Just ask any of them: "If I asked the other man to point to which box the white ball is, to which of the two boxes would he point to?" 

Any of them will point to the box in which the black ball is.

This riddle serves to prove that since a liar to lie has to have a base of FACT to be able to construct its opposite, a liar is always an excellent source from which obtain the truth from.