adding to this to say that a subjective narrator (most narrators fall in this category) is the conduit through which the reader discovers the story. it will never make sense for the subjective narrator to have ALL the information immediately — as they discover things, so does the reader. yes, the reader may realise things before the narrator, and that’s okay! it still doesn’t make them unreliable. remember that you (the reader) are not within the narrative, and are able to step back and view all the details in a way the character can’t.
in some cases, this is totally intentional! when the reader/audience knows something that the narrator (or other characters) doesn’t, that is called dramatic irony. it is a device most often used in the theatre, but can be used in books as well. again, the use of dramatic irony does not mean the narrator is unreliable!
in identifying an unreliable narrator, ask yourself if the character has an agenda. what would the character gain by manipulating the way the story is told? if there is no agenda at play, then the narrator is merely subjective, not unreliable.