I think you want a single source light (that is, not a multi-directional ring light but more towards the laser end) which you can shine at different angles across the lens. A pure white light, such as from the Nikon, will be refracted by the scratch and make it easier to spot. LED ring lights tend to not have decent light in that respect.
Got a hazy memory that a polarizer can be used - the scratch twists the light so using two sheets of polarizer, with the lens in the middle, can make the light viewed through it dark, except where the scratch lets it shine through.