The most effective way to test the function of a condensate pump is to fill the condensate pan with clean water. This method allows the pump to operate under normal conditions, ensuring that it can properly activate and pump out the water as it would in a real-world scenario. Unplugging and plugging in the pump, disconnecting the condensate drain line, or filling the pump with water directly do not simulate normal operational conditions and might not effectively test the pump's functionality in actual use.