Knowing a Coach bag's serial number allows it to be checked for authenticity. Without this number, it's impossible to know whether the bag is authentic.
The interior serial number in a Coach bag starts with the letters "No," and has four numbers following that. This writing is located on the authenticity tag, not on the bag's fabric. Another clue to the bag's legitimacy is that the authenticity tag is never written in any language except English. The serial number is always capitalized when it contains letters. The tag is always made from leather, not fabric, which occasionally happens among knockoffs.