Turn off city water or your water pump and then look down the toilet with a flashlight. Easy to see the level then. Very hard to see when water is flushing by.
We went for a season and a half with non functioning sensors. I finally cleaned them off with one of these.....
It worked well. I agree with the poster above though that the tank doesn't need to sparkle everytime. I will probably give it a good rinse once or twice a season and leave it at that. If my sensors aren't accurate it doesn't bother me.