How can I find out where someone is buried in Hawkesbury, Ontario, Canada?

Same as you would find out where someone is buried elsewhere - cemetery records, obituaries, funeral records. Other options include OCFA (Ontario Cemetery Finding Aid) and the OGS (Ontario Genealogical Society) branch that covers your area of research. When writing to any society requesting information always include an SAE and 2 IRC's as a courtesy. Not all requests are fulfilled for free so be prepared to pay a nominal fee for their time.