Yes, you can. But this is not a method I would recommend. Because of the clouds, the panel won't get sunlight properly. So, the solar panel won't be able to display its full performance. If you test a solar panel on a cloudy day, the results might mislead you to think the solar panel is malfunctioning. But in truth, the panel is working properly. The issue lies in the low sunlight. A clear and sunny day is the best day to test your solar panel. (2)