Which is a way to tell if a narrator is reliable?

Reliable narrator is the standard form of narration in most stories. A narrator is the voice who is telling a story. That voice can be reliable or unreliable, in that it can be considered to always be telling the story accurately or not. Unreliable narration is really a special, rare case where the narrators credibility has been compromised. To understand reliable narration it is easiest to consider unreliable narration.

A narrator might be considered unreliable if their telling of the story is shown likely be inaccurate. The narrator may give conflicting accounts of events (e.g. "Hero"), be revealed to be lying (e.g. "The Usual Suspects"), or it may become apparent that the narrator though believing their account might be suffering from a mental disorder (e.g. "Memento"). In all these situations the narrative is taking place from the first person perspective.

Note that in the above examples (I used films, but it applies to everything) the narrator is deceiving the audience i.e. if the audience takes what the narrator says at face value then they have an inaccurate account of what took place. This is different from the narrator being either mistaken, or themselves deceived by another character, in which case they are not being unreliable.