There's NO single set of rules that tells you when something's fake. Every single bag or wallet or keychain or pair of shoes has to be looked at individually. DON'T try to do your own authenticating. Something is
probably authentic when all of the details are correct for a bag of
exactly the same style made in the same plant, month and year as another one that's been proven genuine.
And there are NO one or two or three details that can ever prove a
Coach item is genuine, including a "correct" serial number. Why do people who write this kind of nonsense in their useless and dangerous "authenticity guides" think that counterfeiters can't find correct serial numbers??? Don't EVER depend on the nonsense you read in authenticity guides.
Post photos at the Authenticate This Coach thread and follow the directions at the top of the page.