The picture of Quarter Sawn on Wikipedia in my opinion is not a good way for it to be done. To get the most quarter sawn wood, and the least, is to cut the log in half, then cut-side down and then cut 90 degrees to that.
A lot of wood is wasted this way, but for furniture that is the only way to go. White oak is most always quarter sawn.
The easy way to tell, like shown, is that the annual rings will be parallel to the sides, where as flat sawn, they are circular in shape.
This is incorrect, if you are referring to quartersawn. As shown with Doug's blank and the Fox, in quartersawn the growth rings are perpendicular to the face of the board/blank, not parallel.