don't discount a dogs ability to judge people. iv'e had a few through the years & can tell you they can. every lab I've had has exhibited this trait. most reciently my gal exhibited a strong distrust of a cable-man. never seen her angry at anyone, loves kids. the most gentle thing. got her as a pup & she's seen nothing but love but she wanted to tear this guy a new ass.
first lab, ABBY, loved everyone, watched over my boys when they were young. nothing got by her. only happened a couple of times but she set herself between my boys & others with teeth drawn. set her self infront of the wife at times.
my boy brought home a friend once that my dog didn't like(out of all of them it picked this kid) told the son not to be around this guy & don't allow him at the house. that boy ended up in juvy.
every dog iv'e had has exhibited this trait & I will trust my dogs intuition.
Ill trust my dogs.