A judicious person is someone who exercises sound judgement. This fits the
OP's description of someone who trusts the trustworthy and distrusts
the untrustworthy —in other words, merited trust. Judicious also seems to be a reasonable midpoint between naive and cynical.
judicious (adj.)
Having, exercising, or characterized by sound judgment m-w
Having, showing, or done with good judgment or sense. Lexico
As important as trust is, it needs to be earned. There is nothing
praiseworthy about trusting foolishly when we have no reason to do so.
Though it is always preferable to expect the best of others rather
than the worst, our trust should be judicious, not naive. T. D.
Williams; Can God be Trusted?
"Authentic trust is a judicious combination of trust and distrust, superior to blind trust, which is foolish percisely because
it bars distrust from consideration..." R. C. Solomon and F. Flores,
quoted in A. Cerra and C. James; Identify shift
By "trust indicator", we mean some quality of an agent which counts as
evidence that they should/should not be trusted. For example, when
choosing to hire a plumber, recommendations from friends, online
reviews, and a quality web site with clearly indicated prices may all
be relevant indicators of trustworthiness, which the judicious
homeowner will combine into a single implicit score, effectively
inducing a ranking among candidates. C. Goutte and X. Zhu; Advances
in Artificial Intelligence