To be certain, you should:
* Consult your owner's manual: This is the best and most reliable source.
* Look under the hood: Systematically examine the engine bay, focusing on areas near the radiator and engine. The tank is usually clearly labeled.
* Use online resources: Search for "[Your Year] Ford Taurus/Mercury Sable coolant reservoir location" with images on Google, YouTube, or similar sites. This can give you visual clues.
Remember to always check the coolant level when the engine is cold.