Firstly, you cannot calculate the light output purely on the DC input power to an LED. The missing factor is its efficiency which varies greatly across products and design purpose.
There are other factors to take into account - the spectral response (the output at different wavelengths), the radiation angles (the output in different directions) and whether these vary under different drive conditions. If you just want a 'general' idea of brightness, for example to match LEDs of the same type you can use a CdS LDR which has a fairly broad spectral response and poor directivity. Just measure its resistance at a fixed distance and angle from the LED. If you need a more scientific measurement, first narrow down the viewing angle and be prepared to take many measurements from different angles and do it through a calibrated sensor, using filters if necessary.
It is quite a complicated task to derive a single figure result!
Brian.