Vegetius\' description of a Roman Legion

[M. Demetrius]

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.