How can I bench test a Flame Sensor (Flame Rectification)
The 'sensor' itself is just a wire. If it is positioned correctly in the flame, connected properly and not shorted to ground it will work, period. The circuitry should (with very, very high reliability) cause a shutdown if the sensor is open or shorted. 

Testing the circuitry is much more involved, particularly if you need to verify the fail-safe and redundancy characteristics that are required for safety. If it is faulty it should be replaced in accordance with the manufacturer's recommendations and local codes . 

For those unfamiliar with the 'flame rectification' method of flame detection, the 'sensor' looks like this: 



The 'flame rod' is a metal wire that extends into the flame The mounting bracket has a ceramic insulator and the flame rod has a spade connector for a wire running back to the electronics. Such an electrode extending into an open flame has diode-like characteristics. The asymmetric conduction (between the rod and the burner, which is grounded) is used to distinguish between shorted, open and resistive conduction (as might be caused by faults or condensation or contamination). 

It is extremely important in gas combustion systems to avoid a false positive flame detection output, since that would allow the gas valve to remain open in the absence of a flame. The gas would then continue to flow, mixing with air, until it finds a spark or flame to ignite it, and 'kaboom', as happened recently here (Photo from the National Post):