If someone was an Once-returner in last life how can he know that ? There are 4 stages in the path to nirvana.  one of them is called "Once-returner" and they will be born only once as a human. So my question is , If Someone was an once-returner in last life , how can he know it in this life ? And if he does nothing in this life to attain nirvana, will he automatically be arahath and no rebirths?
"So my question is , If Someone was an once-returner in last life , how can he know it in this life ?"


When one has attained to stream entry and the higher stages such as Once-returner etc. both ones morality and spiritual faculties (indriyas) are well developed and firmly established. 
When they are mature and well developed we call them the Five Powers. They work in unison each balancing and strengthening each other further.

This creates a powerful momentum and that momentum might mean that a Once-returner does not have the possible to "stop" on his/her path. That being would be so inclined towards Nibbana that anything else is not possible.

A Once-returner has done away with 3 of the lower 5 fetters and have weakened fetter no. 4 and 5. Especially the removal of fetter no. 2 (doubt in the Buddha) is important. A Once-returner will have full confidence in the Buddha and his teaching as he/she has already destroyed fetters and entered into the noble path.


  "And if he does nothing in this life to attain nirvana, will he automatically be arahath and no rebirths?"


I'm not sure its possible to do nothing once one has entered into the stream.

Either way the Once-returner still needs to practice in order to remove the 5 higher fetters thereby attaining to Arahantship.

A Once-returner will be reborn one more time into the Sense-sphere Realm which contains; the Hell realm, Pretas, Animal Kingdom, Asuras, Human Realm and Deva Realm (heavenly realm).

As the 3 lower fetters has been destroyed a Once-returner can only be reborn in either the Human or Heavenly Realm.