1997 Geo Prizm check engine light and bad fuel economy

My check engine light came on and I had the fuel sensors checked and the technician cleared the codes and turned the light off. I can't remember if he told me if the upper or lower sensor needed to be changed because when I had another technician check this he said we have to wait until the check engine light comes back on to do a check because the light was turned off and the codes have been canceled. Is this correct or is there another way to check these sensors? My car is using a lot of gasoline.