I mean a mentor that actually judges people they mentor on accomplishments. It may be like "Are you doing the work or not?". I also mean one that doesn't automatically assume that someone who talks on and on about a topic knows as much about the topic as they might think.
A mentor who not only knows how to mentor those who don't see that the other person is uninterested but also know how to mentor those who (also) speak with authority on special interests, not seeing how poor their grasp of their interests are.
And finally, a mentor who says so, clearly if:
*They see a flaw in the thinking of someone they mentor.
*They think someone they mentor has missed a key point, is not clear, or just wrong.
*Whoever they mentor is using bad spelling and/or grammar (punctuation and use of letter case) to the point that others have trouble understanding.