I have UV lights in the room, this wil show a fake in an instant. I do not hold the money up to the light but open the envelope and look inside or count the money. A fake stands out a mile, although more ?20.00 fakes than anythiong else and if a customer is paying me with 2 x ?50.00 and 1 x ?20.00 I can bet my bottom dollar he has had the fake passed on to him and it is a ganuine mistake.