O2 sensor can be tested by an obd2 based scan tool and I hope that Scan tools are available with many experts in the trade including Agents.
Actually if it is faulty MIL indicator should come up and there should be some fault code related to O2 sensor and scan tool won't be necessary.
I have got my O2 sensor tested by a scan tool from a professional and he confirmed that my O2 sensor is operating within the parameters.But he wanted to compare the readings with another car (same model) with a good sensor to confirm the condition of the sensor.So I have understood that when the sensor is weak it is very tricky to identify because most of the scan tools cannot see the difference between a good one and a weak one.A weak O2 sensor can be the reason for low fuel consumption and some times you only have to replace the sensor to identify the fault and O2 sensors are expensive too.
By monitoring the sine wave out put of O2 sensors using some scanners it is possible to see the difference of the sine wave ( distorted wave form)generate from a bad sensor when comparing with a another car with a known good sensor.So still I am not sure about the O2 sensor of my es5 and still waiting to know the reason for low fuel consumption.