"How would we know if the principle is correct?"

The question is asking:

By what criteria could we determine whether this principle is justified?

For example, suppose someone proposes the principle:

"Present stewardship should outweigh ancient occupancy."

A critical reader asks:

What reasons support this principle?

Are there counterexamples?

Does it apply consistently?

Is it preferable to alternative principles?

So the question "How would we know if the principle is correct?" is equivalent to:

"How would we determine whether the principle is justified?"

It is not asking merely if the principle is true; it is asking what standard or method would allow us to evaluate its truth or justification.