[header] How to identify oaks by the acorns [title] Examine the acorn cup scales. [step] The nut of the acorn grows from a woody cup, which may remind you of a hat. The small scales that make up the cup may be thin and flat, or thick and forming wart-like growths (tubercles). 
This variation is a great start for narrowing down the possible species. [substeps] All oaks native to north america and europe have a spiral of overlapping scales on the cup.