Set up practical equipment with a multimeter connected to a solar cell. Place a lamp to shine on the solar cell and make sure you have a large enough piece of paper to cover it. The paper should be opaque.
Cover over the solar cell and record the voltage on the multimeter.
Slowly uncover the solar cell and record the voltage at each step.
If you have time repeat each recording to make sure you have not made any errors.
Watch out for desk lamps with metal shades as these can get hot.