How should the coolant level be verified when the engine is cold?

Checking the coolant level in the reservoir sight glass is a straightforward and effective method to ensure the coolant is at the proper level, especially when the engine is cold. The sight glass provides a clear visual indication of the coolant level without the need for any additional tools or measurements. This method helps prevent burns or injuries that could occur from opening a hot radiator cap and avoids the risks associated with inaccurate measurements from other tools.

In contrast, measuring with a ruler may not provide an accurate reading due to the shape and design of the reservoir, which can complicate proper measurements. Looking for condensation inside the cap is not a reliable method for determining coolant levels, as it doesn't provide a direct measure of the coolant in the system. Using a thermometer is also irrelevant for checking coolant levels; it is intended for measuring temperature, not fluid levels. Therefore, using the sight glass is by far the most efficient and safe way to confirm the coolant level when the engine is cold.