Coolant leaks are detected either by pressurizing the system or via installing a dye in the system. Cost $45
Coolant leak can be any at place where ever the water travels in the motor. There are external leaks and then there are internal leaks inside the motor. External leaks are not as costly as the internal leaks. As the additives in the coolant deteriorate, the coolant no longer can inhibit rust and contamination. When this occurs the rubber hose connections and gaskets start to leak and rust starts to build in the cast iron motor. Rust in the motor acts like cancer. If you are adding liquid to the overflow container, the system has a leak. Coolant leaks may or may not be visual.