Sentence:  [The "Tutor"] should accustom him to make as much as is possible a true Judgment of Men by those Marks which serve best to shew what they are, and give a Prospect into their Inside, which often shews it self in little Things, especially when they are not in <WSD>Parade</WSD>, and upon their Guard. 
Meaning of the word marked with <WSD> tags:  Posture of defense; guard.