According to Matthew 5 vs 6: blessed are they which do Hunger and thirst after righteousness; for they shall be filled... My question is how can one be sure in his heart that he is hungry after righteousness...

So simple, just apply this verse to a human being who is hungry and thirsty for food. What does that person do @ that time when he's truely hungry ?

Noting in this world matters to the person anymore than to see food and water to quench the hunger and thirst,

That's exactly what jesus is saying here that when you are hungry and thirsty after righteousness, at that time noting in this world will call your attention like Studing THE BIBLE, PRAYING, DOING IS WILL and wanting to know more of him.

here's how i understand this passage of the scripture.

feodan:
So simple, just apply this verse to a human being who is hungry and thirsty for food. What does that person do @ that time when he's truely hungry ?

Noting in this world matters to the person anymore than to see food and water to quench the hunger and thirst,

That's exactly what jesus is saying here that when you are hungry and thirsty after righteousness, at that time noting in this world will call your attention like Studing THE BIBLE, PRAYING, DOING IS WILL and wanting to know more of him.

here's how i understand this passage of the scripture.

Lovely Analogy.

But the question is not so much how you are fed when you are hungry... or how you quench your thirst.

I suppose the question the op was asking is how the hunger and thirst comes about in the first place. Sort of, how do you develop such hunger and thirst.

They are both mutually exclusive.