1997 Geo Prizm Check Engine Light

My check engine light came on and I had the fuel sensors checked and the technician cleared the codes and turned the light off - now I do not 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 cancelled. Is this correct or is there another way to check these sensors. My car is using alot of gasoline.