It should have a sticker on the side stating the name of the manufacturer and year built. You can also try looking up the manufacturer's website (if they still exist) and use the serial number or product number to look up the model and year built. If none of those work you might want to invest in a professional coming out to give it a once over. Another way is to check with the inspector you used to purchase the house they should be able to determine this as well. They may actually have it noted in their inspection file which you should have received a copy of or, if you do not have a copy you can ask your Real Estate Agent for a copy of your file.