A few weeks back, I came across something I wanted to post here for future reference. The following core principles of source criticism were formulated by two Scandinavian historians, Olden-Jørgensen (1998) and Thurén (1997):
1) Human sources may be relics (e.g. a fingerprint) or narratives (e.g. a statement or a letter). Relics are more credible sources than narratives.
2) A given source may be forged or corrupted; strong indications of the originality of the source increases its reliability.
3) The closer a source is to the event which it purports to describe, the more one can trust it to give an accurate description of what really happened.
4) A primary source is more reliable than a secondary source, that is more reliable than a tertiary source and so on.
5) If a number of independent sources contain the same message, the credibility of the message is strongly increased.
6) The tendency of a source is its motivation for providing some kind of bias. Tendencies should be minimized or supplemented with opposite motivations.
7) If it can be demonstrated that the witness (or source) has no direct interest in creating bias, the credibility of the message is increased. (source)