How does Blizzard determine if I preordered a retail copy of Diablo 3: Reaper of Souls?

If you redeem your CD key / activation key before March 31st, you will get the pre-order bonuses. A member of my online community bought the game digitally yesterday and got the wings without pre-ordering.

Technically if you went to your local retailer and bought a copy before March 31st you'll still get your pre-order bonuses.

TL;DR: Anyone who redeems their activation (retail or digital) key before the end of March will get their pre-order bonuses.

Additionally, if you pre-ordered a physical copy of the game but didn't receive it before March 31st, you can receive the pre-order items by submitting a ticket to Blizzard support.

In Germany we don't have Best Buy but I preorder my games at Gamestop. There you get an piece of paper with an one-time-use code inside the game's box or they hand it out when you come to get your game at the release date.

I think Best Buy would do it in the same way. I'm pretty sure the game will include an one-time-use code.