The best method is under a scope using cell structure comparisons contained in botanical keys, but if all you need to know is whether or not you have oak suitable for use in boats, you can accomplish that just as well using a razor blade and 10X hand lens. The 10X hand lens used in sunlight will provide much greater definition than these photos.
What makes the White Oak group of oaks suitable for marine use (and oaken barrels), is their large, earlywood pores (the fresh, new wood formed each Spring) are clogged with whitish, waxy tyloses that water won't penetrate. The Red Oak group of species lack these tyloses, allowing water to penetrate and the wood to literally weep and seep at the end grain. This combined with other factors causes the Red Oaks to be subject to the rapid onset of mold, fungus and rot.
White Oak end grain. Note the whitish tyloses clogging the large, earlywood pores forming the pith or inner edge of each annual growth ring.
More White Oak end grain.
Red Oak end grain. Note the relatively clean earlywood pores.
A home method to test for Red Oak is to cut it into short soda straw sections and see if it will blow bubbles into a glass of water. This generally isn't a very reliable method because sawdust from crosscutting the sample often clogs the pores. If you use it, then slice away the end grain and any sawdust contamination first using a razor blade. The 10X hand lens is more reliable.
Blackjack Oak. In the Red Oak group but here the pores are clogged with sawdust, not tyloses.
More Red Oak lacking tyloses.
Continued......
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Originally Posted by Gary Breckenridge
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