Corkey, look at the back of the speaker and see if there is anything stated there, like a make and model for the sub, or it's wattage. it should be stamped into or printed on a sticker covering the magnet. Considering there are literally thousands of different 15" speakers, and no two are the same, we can't tell you a thing without more information from you.
People, please think before posting a question, then offer as much information as possible to help us help you. God is in the details, as they say.