Mostly it's just familiarity with the styles and the color combinations, and checking the fonts of any Signature Cs on the charms. There are no simple Rules on fake keychains. The best thing to do is ask here, both BeenBurned and I are familiar with most
Coach styles and details.
The important things to remember is that all genuine Breast Cancer Awareness keychains ONLY were made in a combination of Pink colors, any other color combinations are fake. And fire hydrants on the Pet Mix keychains were ONLY red. But there have been close fakes that came close to the right color combinations, so always ask first.
It's REALLY important to avoid fake keychains - like the cheapo Chinese ones that Wal-Mart and other stores had to recall a few years ago, fake name-brand keychains are usually LOADED with lead and even arsenic and other toxins and no one with children or pets should ever have anything that dangerous in their purse or house. Babies especially love bright shiny colors and like to chew on them, and that could be extremely dangerous.
Coach's plants have to follow laws regarding toxic materials - the counterfeiters don't care.