You can usually tell from skimming the blog post / looking back at the author's history of posts - it's hard to put it into words.

I suspect quality means different things to different people and even changes based on context for the same person.

For instance, a blog describing some arcane linux command to run to fix the issue you're having does not need to be the same level of quality as something you're reading to learn + get in depth info about a topic you know at a very shallow level.