There are two measures, the power in watts, or the luminous intensity, that is how bright is seems to the human eye. There are many optical power meters you can get to measure the total power, You have to apply correction factors to determine the luminous intensity.
This might be a stupid suggestion, but why not check with the manufacturer for luminosity per current. Maybe you dont know what LED's u got...?
Well, if u do, check with manufacturer.
There are a lot of ways to do this cheaply. Do you need to make quantitive measurements or just comparative measurements of the same LED from one moment to the next?