How do you know if you have attained Nibbana? You read the suttas, understand their meaning, and put the teachings into practice. At the end of the day, how do you know if you have attained Nibbana? I think this is an important question because Nibbana is the ultimate goal and you need to know if you have finally reached the end.
If the mind has reached final Nirvana, nothing in the whole world can bother it and make it suffer. 

The mind will completely cleansed of greed, lust, anger, hatred, irritation, delusion, attachment, fear, worry, hurt, sorrow, self-view, etc.