Make an iron oxide solution by putting a piece of steel wool (OOOO is good size to use) in a jar of white vinegar. You could get by with a small piece of OOOO steel wool in a cup of vinegar. Vinegar dissolves the steel wool completely in 6-7 days but 1 or two days is all that's needed for identification purposes. Iron oxide will turn white oak black. Red oak will hardly change color at all.