You cannot measure S/N of a sensor, I think. You may measure a S/N (or (S+N)/N)) of an equipment which measures the sensor's output (ie voltage). The S/N specifies how much the noise of the equipment contributes to the actual signal at its input. Thus it specifies the equipment (ie. an amplifier, an adc, a receiver) sensitivity for a specific S/N ratio (ie 10uV for 10dB S/N).
You may measure a noise profile of a sensor, though. For example a photodiode - you can measure a power spectral density (at specific conditions) of the noise current of the photodiode (ie. nA/sqrt(Hz)) - ie. between 1Hz and 1kHz..