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I think there is a big difference between brutal honesty and rudeness. I am naturally a brutally honest person and it can rub people the wrong way but I know the line between being rude and being honest. There is also a difference between polite honesty and brutal honesty, the difference really is in how you say something. From what you gave as examples I would just consider that someone being rude out of jealousy or fear of losing a friend which is not brutal honesty at all because they are in effect saying a lie out of anger or jealousy.

In my opinion they weren't being honest so I wouldn't call that brutal honesty. When I tell people something it can definitely rub them the wrong way and at times can be considered insensitive but I am never outright cruel or rude nor do I make faces, I just state a fact like I would if I said the grass is green. That is usually how you can tell the difference between someone being honest or lying is if they just state it like a fact and don't overexaggerate it.
Whether someone is being honest or not is all about how they say something and their reactions as well as the kind of vibes you feel from them. Your instincts can usually steer you right when trying to determine truth from lies unless they are an extremely good liar but most people are not.

Also I rarely take someone saying a polite comment seriously for the mere fact that when I don't like people I am usually unfailingly polite to them (They tend to realize I don't like them easily enough by my body language so I don't have to come straight out and sound rude by saying I don't like them), people tend to be polite in uncomfortable situations when they don't like someone but don't want to be rude. So in my experience politeness is usually much more of a lie than someone just being completely honest with me.