How can we be so sure that these new "gospels" aren't true?

Read Jesus Out of Focus, if you want a concise, focused reply to the surge of new theories driving The Da Vinci Code and The Gospel of Judas.

Here's an excerpt...

But they fail to mention that while most of the recently discovered Gospels will claim to come from an apostle (such as Mary or Peter), virtually every scholar knows these claims are fictitious. Moreover, these Gospels are not easily dated. When someone claims that, say, the Gospel of Thomas or the Gospel of Judas is "late first century," we are merely hearing conjecture.

Furthermore, the early church was well aware of these writings and understood that they offered a view of Christian faith utterly different than the genuine apostolic Gospels. Christians of the time did not see these Gospels as rivals. They simply saw them as wrong in every respect: They presented an understanding of creation, humanity, Jesus, and salvation that significantly departed from what Christians had believed from the very beginning.

Also well worth reading:N.T. Wright Debunks The Da Vinci Code, right here, at Seattle Pacific University.