I have heard it was the opinion of others that it might be distinguished by the party's breathing upon a piece of glass, where, the breath condensing, there might living creatures be seen by a microscope, of strange, monstrous, and frightful shapes, such as dragons, snakes, serpents and devils, horrible to behold. But this I question very much the truth of, and we had no microscopes at that time, as I remember, to make the experiment with.
For a start, they were not completely lacking in any understanding. Here is Chaucer speaking through the host of the Tabard, in the Prologue to the 'Cook's Tale', most of which is sadly lost to us.
Mr Hobbs told me that the cause of his Lordship's death was trying an Experiment . . . it came into my Lord's thoughts, why flesh might not be preserved in snow, as in Salt. They . . . bought a Hen, and made the woman exenterate it, and then stuffed the body with Snow, and my Lord did help doe it himselfe. The snow so chilled him that he fell immediately ill . . .they put him into a good bed, warmed with a Panne, but it was a damp bed that had not been layn-in in about a yeare before, which gave him such a colde that in 2 or 3 dayes . . . he dyed of Suffocation.