You can use FB.getLoginStatus to retrieve information about the logged in user. It returns a response object for the user. If the user has authenticated your application, the response object will look like this:
{
status: 'connected',
authResponse: {
accessToken: '...',
expiresIn:'...',
signedRequest:'...',
userID:'...'
}
}
You can use the UserId returned in this object to verify the user's identity.