DeHe - doing well socially with adults but not so much with children can be a red flag too, any social skill deficits in the child are propped up by adults' greater skills (and greater tolerance). It certainly was in our family. Or even being more likely to play with older or younger children, same thing - older children's skills will prop up the situation, younger children may look up to the older child or just be a better match socially.
BUT as I said my 2nd child will opt out of play she's not interested in and we have concluded that she's absolutely not on the spectrum. Watching her play with a child who is a good match for her they will play freely, fluidly and flexibly with neither child constantly dominant or leading the play and she's capable of doing this for hours or even days. But a kid she's not that into playing something she's not that into? "Oh look there might be lizards under those rocks..." and she's gone. Yes she will pick and choose, but she is fully capable of following someone else's lead, and playing in a truly equal and interactive way. Unsurprisingly the kids who are a good match for her are age mates, or close to age mates who I either know to be gifted or who I place bets are gifted.
MountainMom - having read your other thread I want to really strongly encourage you to find someone who is an expert in girls who are gifted and "quirky" I really do think there is something going on with your DD, but it should be a professional, not the internets who help you figure out what that something is. I highly recommend books like "Aspergirls" and "Girls Under the Umbrella" though.
A checklist is far from definitive but these lists were helpful to me in deciding to pursue professional help:
http://suelarkey.com.au/media/Tip_Sheet_-_Girls_with_an_ASD.pdfhttp://www.help4aspergers.com/pb/wp_a58d4f6a/wp_a58d4f6a.htmlhttp://aspergersgirls.wordpress.com/2012/02/10/aspergers-traits-women-females-girls/
