If you are visiting rural Canada and staying somewhere, It’s as simple as asking. Rural Canada covers a lot of real estate. As Thomas WIlliam Volinchak a professional in water treatment, caution in a place you are new to is prudent.
Municipal drinking water in Canada is enforced by municipal governments which are regulated under provincial laws. The provincial and the federal governments do work together. First Nations under Federal Law and funding manage their own drinking water.
Drinking Water - Water Quality
Federal-Provincial-Territorial Committee on Drinking Water (CDW)
Due to the low population density, many people in Canada choose to maintain their own household well and take on the responsibility for their water safety.
There have been a few cases where communities have to deal with short term issues. Bursts from land slides, runoff, or unusual warm weather is not uncommon. Competent and routine testing catches these problems. There have been other rare cases where communities have dropped the ball. If such a case exists in an area, the precautions are well advertised in the area.
Then again, where I grew up, I often drank from streams and springs near roads when on cycling trips. It was probably unwise. Never did experience beaver fever first hand.
There are more local fluctuations in drinking waters in Canada than in the US. I don’t believe that Canada has an equivalent of a Clean Water Drinking Act, and in some of the rural areas, caution should be taken. In the major cities? I would be completely comfortable with the water supply.
Tap water in most areas of Canada is far far superior to the crap you get in bottled water. Absolutely you should refill your bottle from the tap, and feel free to take some home with you and let your friends taste real water for once, not some stale bacteria-laden slop that's bottled in a factory in some industrial town in France.