Now that lavender will be introduced to faverolles here is one way to determine if a bird is lavender or splash (blue).
Look at the feather shafts of the wings and tail. If the shafts on the top (outer side) of the feather are light and the shafts on the bottom (under side) are dark it is a lavender. If the shafts top and bottom are the same color, or a mix of light and dark, the bird has blue (splash, blue, lavender splash or lavender blue).
I hope that made sense.