Had some useful discussion with Amazon Customer Services and essentially they could not find out. My research indicates that serial number will begin with 21,22 or 23 if new sensor fitted.However ,there seem to be further problems with this camera with dust entry into lens system.
I decided to go with the Canon G12 AS A PROVEN PRODUCT.
I had a similar conversation today. I was told that there's no way of telling. I let them know the details about the serial number indicator. I would suggest not ordering until amazon can make a definitive statement. I've ordered from park cameras in the end. It cost more, but they could guarantee their stock had the new sensor.