Those that show faith in Christ. That's it. Sometimes it's not apparent for a long time. If we WERE chosen before the creation of the world like Eph 1 says, then it is inevitable that we will eventually become a true Christian. We can use our discernment to see those that have chosen a life course that SHOWS that they are saved.
But how am I to know that the most militant atheist will not one day come to Christ? Lee Strobel was a huge atheist. But he eventually was called to Christ. So I cannot stand in judgement of anyone.