If you’re really suspicious (and have a UV light handy), shine it on the note. The number “5” and the thread running through the note will glow a bright red or orange. Fakes tend to glow weird colours—like purple or blue—or just stay dark. It’s a simple trick that weirdly feels like science class.
How To Spot A Fake Note Nz at Janice Harvell blog
If you don’t have a UV light, no sweat. You can also hold the note up to a strong light and look for a shadowy image of the Queen that appears in the clear window when viewed from the back. It’s like a watermark, but way more high-tech.
