In advance, no, they don't tell you.

However, up until now every SSD Apple has shipped has been either Samsung or Toshiba, and you can find out which it is by opening OS X's System Profiler. Look under "Serial ATA" and you'll see the drive's description, which will be in the format "APPLE SSD (manufacturer code)(capacity)".

Toshiba drives will have a manufacturer code of "TS" (so my 256GB Toshiba SSD shows up as "APPLE SSD TS256A", for example). Samsung drives will have "SM". When I was shopping around last year trying to find out about this, it seemed that most of the 128GB drives were Samsung, most of the 256GB drives were Toshiba. What they're using for the 512GB drives is anybody's guess, but System Profiler will be the way to find out.