Login Issues

Hi,
Team ,
In Mobile Application  I Gave a Two types Of logins like Mobile number And facebook id . if user is  first time login with mobile number . after that again come with registered  mobile number .. then i will check that mobile number is registered are not .  based on  my conditions redirect to some other page ..
But already user registered  with mobile number .. again come with fb for  logins.. How to Check If That user is already registered are not ...? in this condition i treat like a  user registered with mobile number and facebook treat like a single user

Manikanta