I asked an acquaintance when she knew her son was grammar school material (super-selective county, technically speaking, and he is at the REALLY super-selective grammar). She told me it was when, in Yr1, he was hopping along the alphabet-themed design in the school playground, naming a country for each letter.
I gave up on DD at that point ... nothing really shook my thinking, though I thought maybe she should give 11+ a punt, until October in Yr5, when we found out she had the highest CATS score in her class, and the highest NVR in the year. So she did the 11+ ... didn't pass for THAT grammar in the end, but did pass for the others we shared the results with. We'll never know if she was really grammar school material though, as she's not going to one.
She's in the middle set for maths (top set last year, floundered massively), entered for L6 SPaG only, isn't one of those children who questions everything, develops an intellectual passion for something ... just still isn't what you'd think of as GS material. But she's a lot brighter than she normally lets on, and shit-hot at exams. 
On the other hand, my friend did laugh at me when I was surprised by the CATS results and said 'what, didn't you realise?' so maybe it's just that I'm a bit dim.