I think sometimes it's harder than others. For example, I can read Mercedes Lackey's Take A Thief and enjoy it very very much while recognizing it has serious defects (most notably, it blows continuity for other series in the same world rather sharply, and for bonus points the change serves no real purpose in the story!).

But sometimes, you can't know, especially depending on how you read things. I have to say that it sounds like p_zeitgeist puts a lot more thought into reading fiction than I do. I read things, if I don't enjoy them I toss them aside, and I don't really worry about whether it's objectively "good" if I didn't enjoy it. Sometimes it will leap out at me but usually it won't. I do occasionally say "I know a lot of people like it" if I do, but I'm not always sure whether it's good or bad just because it's liked (which can be a taste thing).

Ultimately, I don't worry about it too much, except when I'm identifying a book that I enjoyed that I do realize isn't objectively a "good quality" book as far as writing. If I miss some, oops. That's life. But if I know it, I'll usually put a disclaimer on it so as not to mislead people.

Well, and then there's the occasions where the author descends to a level where you want to enjoy it but can't. I had a great deal of entertainment out of Lackey's Reserved for the Cat but I'm afraid a decent chunk of that entertainment was in deconstructing all its failings....