How to Tell if You’re a Christian

How can you tell if you are really a Christian? Afterall, none of us is perfect; no one follows Jesus flawlessly. Yet, the Apostle Paul was sure that God had chosen the Thessalonians. How could he tell? How can we tell? I remember the first time I doubted the reality of my own conversion. I was fourteen years old and could have used some teaching from 1 Thessalonians chapter one.