Mellifluous

They use always and never statements. "ALWAYS do this" "NEVER do that" good writers know almost everything works if you're crafty enough, and good writing advice will not simplify things into such black and white terms.

When you ask for a reason, they can't give you one. That tells you they don't know why and they're just copying what they've heard from other writers. They could easily be spreading nonsense.

They repeatedly state their opinions as facts. Example: they don't have to like YA, but they shouldn't stick "YA is immature and stupid and no one should write it" in their advice. I say be cautious if they repeatedly do this, since everyone does it sometimes--but they should be able to step back and be relatively neutral if asked to be.

They talk about things outside their experience/qualifications. If they say they don't understand grammar, don't listen to their grammar advice. If they don't like fantasy, don't listen to what they have to say about fantasy. If they rarely interact with 1st person, don't follow their advice about writing in 1st person. Etc, etc.

You don't like the way that they write the thing. If you've read someone's dialogue and it's not for you, don't follow their advice on how to write dialogue.

You've found bad writing advice from them before. Chances are, if someone was wrong about the things you already knew about (pacing, story structure) they're probably wrong about that thing you haven't learned yet (character dynamics, shock value.) This isn't always true! Some people are good with one thing and bad with another. But do consider if you're happy with their track record.

The character says that there’s no white pencil.  But you don’t need a white pencil to draw a white egg.  We already know the egg is white.  What we need to draw is the luminance of the yellow lamp and the reflection of the blue cloth and the shadows and the shading.

We know a broken bone hurts.  We know a knife wound hurts.  We know grief hurts.  Show us what else it does.

You don’t need to describe the character in pain.  You need to describe how the pain affects the character - how they’re unable to move, how they’re sweating, how they’re cold, how their muscles ache and their fingers tremble and their eyes prickle.