to be honest i have no idea how much advance you get as the revs rise. I can see on my engine because i can look at the ecu, but i dont know about standard bosch setup. obviously at idle it should be spot on the marker. Then atleast you can tell if its retarding if the marker blurrs in the wrong direction as you rev the engine.
If its advancing the marker will blurr towards the bulkhead, if its retarding it will blur towards the front of the car.
To be honest if the knock sensor was faulty you would get an engine management light, and youd feel a loss of power if it was retarding.
Engine management light is off. Lack of power - on.
I said "faulty" in terms of too sensitive. Or maybe my engine is too noisy already and the sensor confuses engine noise with pinking. I remember even seeing a diagram (for American car) for desensualizing the knock sensor with additional resistors.