How can I identify if a file is a valid PNG image by its file signature?

Every PNG file starts with an 8-byte fixed signature: `89 50 4E 47 0D 0A 1A 0A` (hex). This signature is a magic number that distinguishes PNG from other formats. Tools like Hex Editor can reveal this signature, and any valid PNG must begin with it. Understanding this signature is the first step when analyzing image files for potential steganographic payloads, as described in the article Steganography Techniques - Hiding Payloads Using PNG File Format.