The similar question was raised by Ales Rarus some time ago. The Bible warns us about false Christ. But how do we distinguish between the real and the false Christ?
Today's Gospel (John 7:40-52) speaks of this very situation during Jesus' time. "Would the Christ be from Galilee? Does not scripture say that the Christ must be descended from David and come from the town of Bethlehem?" Our parish priest, during the homily, said that Jesus was rejected because the Jews knew too much about Him and they were too familiar with the scripture. If you are Frank Peretti's fan, surely you'll know about his novel called 'The Visitation', which depicted the same situation.
Now, back to the main question. If there were someone who claimed himself as the Christ, how would you intend to find out whether he's real? What questions would you ask him? What criteria would you look for? Or maybe, let's twist the question the other way round. If you were one of the Jews in Jesus' time, would you reject him too?
Tough questions, eh? At least they are tough for me. I would most probably ask him this question: Lord, do you still remember what was the gravest sin I've ever commited?
Or perhaps, the only way is to let the Holy Spirit leads.