By post. If they don't arrive, the primary school gets the result. So you could ask current headteacher on the Monday after results are out. (I think schools actually get the results a few days before they are posted!)
JRM is correct.
Our local primaries are very reluctant to discuss results directly with parents, even when results are held up in the post - phone the grammar school if results are late, they are usually very helpful.
I think there have been occasions when primaries have had the results after parents. I think it helps to remember it is the grammar schools' test. They run it and it is probably only as a courtesy that they let primaries know the results.