How can I tell the year of a Briggs&Stratton rider lawnmower engine?

Look for a model & serial number on your machine...either on the engine or somewhere else. Get on the Briggs & Stratton website and see if they have a Parts or Service section. All models/serial nos. are grouped by year of manufacture, so somewhere on their website should be a way of matching up your numbers and finding the year. At the very least, you can e-mail the customer service folks [or use the 1-800 phone number] and ask them to find it for you.