On a scale from naive to cynical, what would be in the middle? [duplicate]

Someone who is naive trusts others without question or examining their claims. Someone who is cynical never trusts anything another person says, even if there's no evidence that they are being anything other than honest.

Is there a word to describe someone who trusts the trustworthy, and distrusts the untrustworthy?

"Street smart" is the only word/phrase I can think of that might fit, but I'm hoping there's something better and less colloquial.

Inspired by this post: On a scale from Optimist to Pessimist, what would be exactly in the middle?

In a word, sensible - having, containing, or indicative of good sense or reason.

Sensible people can make for spirited debate

Or, perhaps, level-headed - behaving in a calm and sensible way, even in a difficult situation.

In those situations, you'll have an awkward road ahead of you, but try to talk with your parents in a level-headed manner about their concerns

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