Take a look at the price range for rental listings in your area. Find a high/low and average depending on the finishes. Use resources like Kijiji, Facebook Marketplace, Padmapper, Apartments.com etc.
If you are using a property manager or know a PM you can speak with, get their advice. They have eyes on the market and are constantly advertising and leasing units.
Trial & Error
Start with the highest rent you think you could achieve, if the ad sits you know your price is too high. Adjust quickly and don’t let ads age.
Real Estate Reporting
CMHC publishes rent reports every year in each major city in Canada. However, it has been widely known that the rents published are extremely conservative.