I wasn't diagnosed until the age of 49. I've known all my life that I wasn't like everybody else, but I made my best effort to fit in as well as I could and behave enough like everyone else to get along. It seemed to work reasonably well, most people didn't treat me like a total freak, and I managed to skate by on the fringes of the peer group - not ostracized, but never fully accepted, either. That was okay, I didn't have the social skills or desires to really be in the middle of the 'popular' group.
Once I entered the work force, I got on okay with my immediate peers, but the more Alpha-management type personalities absolutely hated me - I couldn't keep a job for more than about 15 months at a stretch, even though I was recognized as highly talented in my field. I had a few friends, all of whom were social misfits like myself.
Shortly before my career finally fell apart once and for all, I met a woman at work whom I later married. After I'd been fired from that job and was puzzling over what happened - I had been 'thrown under the bus' by a couple of coworkers I had thought (in my Aspergian naivete) were rather good friends, at least people I trusted. My wife then told me "I'm not surprised by that at all. When I first started working there, every time you left the room they went on about how weird you were." I had known she didn't particularly like these people, but until then I didn't know why. Point being, I think that sort of thing has probably gone an around me all my life and I was just oblivious to it. So I don't think its really possible for an Aspie to know for sure how we appear to those around us.
No matter how 'normal' we think we're acting, we are missing the capacity to recognize a lot of nonverbal social cues, so how can we ever know if we're mimicking all the right ones at the right times? How can you know the people around you aren't sending each other "What a dork!" messages to each other that you don't even notice?
When I was diagnosed, there were a few people I'd know for many years who would say things like: "Really!? Autism? You've always seemed so normal to me."But what that actually means, I've come to realize, is: "Autism? That's retardation - you never seemed ret*d to me."
The people who've known me well for a long time didn't seem shocked at all. It was as if they've always known there was something wrong with me, but didn't have a name for it, so they just said nothing. I can't count the number of people over the years who, trying to carry on a conversation with me while I swayed from side to side and stared at the floor, asked me "Do you have to do that all the time? What are you, Autistic?" Funny thing is, when they asked, they were kidding. When I answered "Yeah, I think so - lil bit." I wasn't.