How do I calculate the illuminance of a laser beam?
As tuskiomi said above: Use a laser power meter, which will give you watts. (A solar cell will not give you an accurate power measurement because it is illuminated unevenly).

Once you have watts, find the luminous efficacy of your laser in lumens/watt. Specifically, you probably want the photopic luminous efficacy, as it is a bright light. This should be fairly straightforward as your laser is (hopefully) monochromatic. Having determined the luminous efficacy, you can now find the luminous flux. Based on the beam area and luminous flux, you can now find illuminance in lumens. Err, and I think you need to multiply by the luminosity function somewhere in there. (There's a reason it's much easier to express laser power in watts).