Well, if you set one up in an integrating sphere, you could get a lumen measurement out of it.
The traditional way though, is to fire one into a laser power meter, set the laser wavelength on the dial to the laser wavelength you're measuring (typically, 650nm for red, and 532nm for green), and see how many milliwatts it outputs.
The poor man's laser power meter is an amorphous silicon solar cell, a DMM set to a low milliamp range, and a set of mathematical calculations that calibrate the solar cell to the laser wavelength, and convert the result to milliwatts. I busted my solar cell hooking it up to a scope about a year ago, and lost the mathemetical formulae in an earthquake in February 2001, so I can't pass along the math or do any power output testing of my own green lasers.
