I notice it in posture and body language. Also, I can always tell from hands. Aspies curl up and hold their hands in a weird way, they have their hands poised in an awkward manner. If you look at the picture Rob has of himself with his wife and kids, you can see that Rob's hands are held in a distorted/ awkward way, while his wife's hands look fluid.
My hands always look freaky in photographs.
Show me a picture with hands, and I can tell you if the person is aspie.