How to field test true wattage from a solar panel?

[Mojave Traveler]

I have a new solar panel that just arrived and I'd like to test to see how close to the advertised output does it actually put out. What is the best way to do this?

 

[smritte]

Point it at the sun directly and measure current. Some of the controllers will show you this. While your there, move it out of align and see how much it drops. The panel on my trailer is about 60% -70% sitting flat on the roof compared to when I pointed it right at the sun.