For example, if she has her phone out, she's looking at her phone and says "wow, my sister's text me five times already" and she looks up and give you a little smile and you grin back at her, if you give her a grin like this (Just the grin, not the eyes or face), do you think that it was to be polite or because you thought what she said was cute?
http://www.istockphoto.com/file_thumbview_approve/5997552/2/istockphoto_5997552-cheesy-grin-man.jpg
http://www.istockphoto.com/file_thumbview_approve/5997552/2/istockphoto_5997552-cheesy-grin-man.jpg