Basically there are a whole host of rules for evaluating evidence, this is known as the Historical Method. In addition to this some biblical scholars use an additional set of criteria for criteria of authenticity regarding the historical Jesus.

Usually the Historical method alone is sufficient to determine the authenticity of something, however, if one wishes to disprove something that is considered proven just saying "I don't belive that" is insufficient, you need to provide a better theory to replace it.

For example, if I wanted to make the case that Jesus or Socrates were inventions of the early church or philosophers, I would have to provide a very solid explanation for how and why it was done. Did Plato, Xenophon and Aristophanes simply sit down and decide "let's make up a man who is the father of philosophy, think up a bunch of brilliant shit he supposedly said and attribute none of it to us even though we're the ones who thought of it and let's do it all for shits and giggles"? This theory in turn would have to pass examination through the Historical method and be a stronger theory (greater explanatory power for equal or lesser assumptions or vice versa) for it to have a chance at being accepted. The same basic case can be made for Jesus.

I'll put a few examples of the methods used to evaluate evidence in a reply to my own post to illustrate a few of the methods used to evaluate various forms of evidence.