How to synchronise with facebook that username & password are valid or not?

I am creating an application in which I am trying to add facebook account in app. What I wish to do is when user enter his/her username and password my application should call some facebook api or any other thing by which it will confirm that the user credentials are correct for faceb开发者_运维技巧ook.

Then the API will return some message by which I can add account or give an alert that credentials are not right.

it is not possible to login using the username/password directly to the api. use facebook connect instead.