The difference between double factor violet and single factor violet can not always be visually identified with any certainty. Sometimes a single factor violet will appear more violet than a double factor and vice versa. The shade is effected by the dark factor so that is more likely to be the difference you can see.
One pair I had that I thought were both double factor violets produced a non violet chick which proved that they were both single factor. The most purple bird I've ever owned was a single factor violet
When two single factor violets are bred together only 25% of the chicks will be double factor violet
I know she's violet because one of her parents were and I now have babies from her and the male shes with who is also carrying the violet factor and so far at least 2 of their 4 babies are Violet and one is a double factor Violet - You can't get a double factor violet if only 1 parent is violet