I suppose we had an advantage, because our son's behaviour was so alien to that of his older siblings.
We might have had him down as 'highly strung' or 'sensitive' but really his behaviour was off the scale.
We were lucky we had something to compare it to.
If you met him now, at 5, he could easily 'pass'. He is articulate, curious, charming.
But utterly, utterly unable to relate to his peers. He loves Lego, Minecraft, dressing up, all things that should be fun, but he doesn't care for the opinions/ideas of others.
He finds it easy to play alone, but is already aware that others are different.
We can see him beginning to 'play a part' in an effort to fit in, but it really is beyond him.
When he was 11 months old, we honestly had no clue. Most children of that age are self absorbed.