Stigmata is not enough, lots of people had or claimed to have had them. Performing Miracles is not enough. Thousands have claimed that ability, and Saints are accepted to poccess such skills. There is a very strict set of events that must occur for the over cautious Catholic church to accept someone as Christ King returned. Not to mention other Christians.
And that of course is "The parting of the skies, the rise of the good and pious to Paradise while the foul and wicked ones are dragged beneath. The Four Horsemen would rise to unleash chaos and suffering across the earth, just as damned souls would walk again followed by hell itself opening to reveal hordes of demons, lead by the son of the Dragon on earth himself! The Anti-Christ! Anathema! The Prince of Darkness! THEN, would Christ descend from Heaven with a mighty Host of Angels to engage in the great battle between good and evil! The dichotomy of existence! The Final and only true Struggle!"
At that point, any christian and non-chrsitian left on Earth would probably go: "Yeah, that's the dude."
Mind you, if he's that good it doesn't really matter if he claims to be the Second Coming or not, does it?