The oil level in transformer bushings is monitored using oil level gauges or indicators installed on the bushing. These gauges provide a visual indication of the oil level, ensuring it remains within the acceptable range. Modern systems may also include sensors that send alerts if the oil level drops too low, indicating potential leakage or oil loss. Regular visual inspections or automated monitoring systems help ensure that the oil remains at the correct level to provide adequate insulation and cooling for the bushing.