Here's what I'm seeing on NTSC wiis based off BocuD's discovery
1) 100-113 is 6 layer, 114+ is 4 layer
2) 2 is all nonwhite
3) 3 is all six layer
4) 4 is rare, with only one white wii recorded, but so far all are 4 layers
5) 5 is all 6 layer
6) 6 is 6 layers
7) 7 is 4 layer
8) 8s are all black
9) 9s are red
Of these, the only ones we need to worry about are the white wiis that are possibly 4 layer. That leaves serial numbers 1, 4, and 7. Serial number 4 only has a few documented consoles, and of those only one is white, however they are all 4 layer so it's a good number to keep in mind. However, the numbers you are most likely to be looking for are serial numbers are 7s or 1s in the 114+ range.