Most pure oils will not leave an oily mark when they are dried. This does not work with citrus oil or absolute oils however. If you know that your oil contains alcohol then you should be able to smell this as soon as you open the bottle.
Most pure EO will not leave an oily mark when dried, however, some pure essential oils will leave a mark, especially expressed oils (citrus) or absolutes. When you add oils to water, the oil should float on the top as opposed to sinking, so this is another great way for you to find out if the oil you have now is what it says it is.