Whether the source’s history or status suggests reliability or unreliability. If we know that someone has told lies in the past, then we should be less trusting of them in future. If, on the other hand, they have been put in a position of authority and responsibility, then this suggests that they have earned it, and so this counts in their favour, to some degree at least.
Ability To See
Whether the source in a position to know what they’re talking about. No matter how honest a source of information, if they don’t have access to the evidence then the value of their testimony is going to be limited. To assess a source using this criterion, consider whether the person was present to see what they are claiming occurred first-hand, and if they were, then whether there were any conditions that might have obstructed their view or otherwise impaired their access to evidence.
Whether the source of information has anything personally at stake. If they might gain something by lying, then their credibility is weakened by their vested interest. If they might lose a lot by being caught lying, then their credibility is strengthened by a vested interest to tell the truth.
There are some situations in which it is difficult for normal observers to accurately interpret evidence because they lack specialised knowledge. For example, if I were to watch a high-level game of chess, my comments as to who was in the best position would be worthless, as even if I were able to see the board clearly I wouldn’t really know what was going on. My credibility about such matters would thus be weakened by my lack of expertise.
Whether someone is predisposed to support a particular point of view for reasons other than vested interest. Someone who knows other people involved in a dispute, for example, may be liable to side with them or against them depending on their relationship, weakening their credibility.