I might be wrong but I believe a lot of it is by color variation, but if I remember correctly Cotton Candy makes things a lot trickier as she seems to have more variations than others, just in my sister's and my collection, not including Nirvanas I believe we have 2 or 3 eye colors and like four different body colors, hopefully someone else will be able to help you identify them better, but I know with her at least it isn't an easy task.