Vegetius\' description of a Roman Legion

[Sean Manning]

I wonder what the decision tree looks like for deciding "here Vegetius is totally correct/here he is entirely speculative". Same applies to Trajan's Column, Adamklissi metopes, Herodotus and other sources. It can't all be tied to the archeological record, as thing like tactics and organization would scarcely be represented by foundations of buildings and escavatory finds. Just wondering as always.

Just brainstorming but:

How much evidence do we have (more evidence -> a higher level of credence is possible if the evidence mostly agrees)
What does other evidence say?
Why did our source say this? Were they an eyewitness? Talked to an eyewitness? Read it- where? Hearsay? Didn't believe it at all but was trying for rhetorical effect?
Does this source describe something which did happen, or say it should happen? Eg. we know that a lot of imperial edicts were ignored after a decade or two; or trying to study the late Roman economy based on the Edict of Maximum Prices.
What hypotheses would produce the evidence we have? Which are simplest?

One source isn't worth much although it can give rise to interesting speculation.