In his Metaphysics, Aristotle says:
To say of what is, that it is not; or if what is not, that it is, is false; whilst to say of what is, that it is; and that what is not, is not, is truth.
I would not normally have suggested that truth was a value, assuming that all held to this; but given the disinformation climate of our times, it seems that truth is a ...
Plato had a simple rule: learn geometry. This of course is well before the 1800s.
This is tempered by the time of al-Jabr who taught us to learn algebra.
Another useful heuristic is by Solomon Lefschetz. Unfortunately this is after your cut off date, being in the early to mid 20C.
He had one simple rule: he said he wasn't interested in prettifying already ...