How do we know anything historically? There is no "scientific" proof that Lincoln was the president. We cannot recreate him in a laboratory or bring him back to life. We cannot reproduce the experiment. We cannot calculate an equation that tells us that he was. But we can assert with a high degree of probability that Lincoln was indeed our president and was assassinated in 1865. We do this by appealing to historical evidence. Many people saw Lincoln. We have some of his writings and even his picture, not to mention his likeness on our pennies. But none of this "proves", in a scientific sense, that Lincoln ever lived or was the president.
The kind of evidence used in historical research is the same kind as that used in a court of law. In a courtroom case certain kinds of evidences are appealed to in order to determine what exactly happened, eyewitnesses are questioned, motives are examined, and physical evidence is scrutinized such as fingerprints or journal writings.
It is the same kind of evidence that we appeal to in order to establish Christ's life, death, and resurrection. Granted, the evidence is not as great as that for Lincoln, nor as recent. But it is better evidence than we have that Plato ever lived, or Homer, or many historical figures that we take for granted.