Well, first, why associate wisdom with peacemaking at all?
Now a peacemaker is one who makes peace, either in himself, or in others: and in both cases this is the result of setting in due order those things in which peace is established, for "peace is the tranquility of order," according to Augustine. Now it belongs to wisdom to set things in order, as the Philosopher declares, wherefore peaceableness is fittingly ascribed to wisdom.
According to St. Augustine's insight on the true nature of peace, "peacemaking" can pretty much be defined as "wisdom at work."
And the rest of the characteristics St. James lists? Well, Rule #1 for doing right is, "Don't do wrong":
... the first thing, to be effected in this direction of human acts [by wisdom] is the removal of evils opposed to wisdom: wherefore fear is said to be "the beginning of wisdom," because it makes us shun evil....
Hence James said with reason that "the wisdom that is from above" (and this is the gift of the Holy Ghost) "first indeed is chaste," because it avoids the corruption of sin....
The rest of the list -- "gentle, compliant, full of mercy and good fruits, without inconstancy or insincerity" -- St. Thomas sees as "the means whereby wisdom leads to peace." Gentleness (which moderates desire) and compliance (to the true wisdom of others) "are conditions required that man may be at peace with himself." The other characteristics are, of course, requirements that man may be at peace with others; being full of mercy and good fruits brings right order to a neighbor's deficiencies.
The Biblical text St. Thomas was working with read that wisdom involves "judging without dissimulation," where the Vulgate reads "without judging, without dissimulation." He understood "judging without dissimulation" to refer to correcting a neighbor's faults in charity, which is certainly a condition of true peace between neighbors.
It seems to me that being "without inconstancy or insincerity" is also a requirement of true peace between neighbors. As long as we don't insist on James 3:17 being a unique and complete enumeration of such requirements, I think we can preserve St. Thomas's idea of the descriptions referring to conditions for peace both within and between men.
All of this suggests that one test of someone's wisdom, a test that doesn't require much soul-reading, is how much at peace he is with himself and with others. (Whether others are at peace with him is less relevant.) Passing the test doesn't suffice to prove wisdom -- fools may well be perfectly tranquil amidst great disorder -- but failing it might just disprove wisdom.