Unfortunately Steven Spielberg's signature is all over the place. There is something about the "S"s that bothers me about this particular autograph. The first one particularly appears to be too symmetrical. The autograph also seems to have a very refined appearance which is not indicative of something that was signed in person. It may very well be authentic. I would just pass on it personally since I do not have a good feeling about it. Take a look at some of the examples here.