Accidental Information. “Paul is trying to tell the Corinthians that Jesus rose from the dead, and he says, ‘He appeared to Cephas’: he tells us by accident that there was a man known as Cephas, and this is therefore dependable. Detection, both criminal and historical, is largely based on this criterion.” (Michael Goulder, “Jesus: The Man of Universal Destiny” in John Hick (ed.), The Myth of God Incarnate (London: SCM, 1977): 48-63, 50).I need to think about this a little more, but I reckon that this is actually a criterion people are often working with without realizing it. The problem with Goulder's discussion is that it is only a sketch, and he gives few examples of this kind of material, where early Christian writers reveal information en passant, but he also points to "the Twelve" in 1 Cor. 15.5 as another example (Goulder, "Jesus": 54). One might add other information that Paul provides in passing, like the fact that Jesus had brothers who were married (where the point is that people have a right to take a believing wife on mission, 1 Cor. 9.5), or likewise that Cephas was married (also 1 Cor. 9.5). And I reckon the criterion could be applied to some of the Gospel material too, e.g. that Jesus had a house in Capernaum (Mark 2.15)?
Update (14 January, 09.48): thanks for all the comments on the post, several of which show that some more clarity is required in defining this criterion. Goulder's point, as I see it, is that a given author will give away information that is shared by him/herself and his/her audience in the course of constructing an argument about something else. So when Paul says "Have I not got the right to take a believer as wife . . . as do the brothers of the Lord" (1 Cor. 9.5), the information here provided en passant is "Jesus had brothers" and, what's more, "Jesus' brothers were married". His point is not to convey this information; he is not narrating this information, or making it available afresh to readers who previously knew nothing of it. Rather, it is shared information that Paul can take for granted, but which gets conveyed (to us as historians) while he is relating other information as part of an argument. To try to make the point more clearly still, Paul's point in 1 Cor. 9.5 only makes sense if it was reasonably widely known that Jesus had brothers who went on mission. The argument takes for granted that there were such people, and the assumption is necessary for the argument to work. It is not the point of the argument.