At the beginning it is always hard to distinguish between auras and afterimages. It is easy to dismiss the beginnings of seeing auras as just afterimages. What you have to do is keep at it and not try to distinguish between the two. At least, that is what I had to do. The difference gradually became clearer. I can't see auras on demand now, but I go through phases when I will begin to see auras clearly around people. You can practice by placing the tips of your index fingers together and gazing at them in an unfocused way. Then, pulling the tips slightly apart, you should see a trail connecting the two.