One of the tools that historians use is something called multiple attestation. Multiple attestation simply means that something is more likely to be historical if it is attested in multiple sources. It is often used to examine the words and deeds of Jesus in the Gospels.
In general (not just when applied to the Gospels), multiple attestation is not perfect. It is possible that something historical was only mentioned once and there could by multiple records of something that was not historical.
Still, multiple attestation is a helpful tool when looking at historical documents.
However, there are some misconceptions. It is not enough to see an event found in Matthew, Mark and Luke (Synoptic Gospels). If an event is first mentioned in Mark and the version shared by Matthew and Luke is based on copying that report, that is considered only single attestation.
If a story is found in Mark and a separate tradition is found in Matthew and Luke (common tradition between Matthew and Luke not found in Mark is often called Q), that is considered two sources.
Things get complicated when people attempt to throw in the Gospel of Thomas. What is the relation between Thomas and the Synoptics? If Thomas used the Synoptics (which is likely), then Thomas doesn’t help.
Historians love when a tradition is also found in John or Paul. Sometimes, stories such as the arrest of John the Baptist by Herod is found in a non-biblical source such as Josephus. There are a number of overlaps between Acts and Josephus, which is also good.
Multiple attestation is not perfect but it is a helpful tool for historians.
