First of all, he never preaches anything but the truth. He preaches the word of God in its fullness. He never undermines the doctrine of the Church or promotes heresy and dissent. He prays the liturgy with reverence and follows the rubrics. He loves the Blessed Mother, guardian angels, and the saints. He knows that love isn't a warm, fuzzy feeling but a strong desire for the greatest good for the other, i.e., eternal salvation. He knows that sometimes love requires speaking the hard truth.
On the other hand, how do you recognize a priest who can't be trusted?