Pay Attention to the “Rays”. When looking at the face of the part, especially in the flat-sawn areas, you may notice little dark brown streaks running with the grain, sometimes referred to as rays. Red oak will almost always have very short rays, usually between 1/8″ to 1/2″ long, rarely ever more than 3/4″ to 1″ in length. (Pictured below on the right.)
White oak, on the other hand, will have much longer rays, frequently exceeding 3/4″ on most boards. (Pictured below on the left.)
This method is probably the most reliable under normal circumstances, and is useful in situations where the wood is in a finished product where the end grain is not exposed.