There are actually several ways to check it. Search these forums; this topic has been discussed extensively.
The easiest way is to go to the System Infomation Energy screen (the one with the outline of the car). The roof of the car in the diagram is actually a graph, dynamically showing if energy is being produced, and how much in relative terms. The sunnier the day the more of the squares on the roof of the diagram will be filled white. It's subtle and many people miss it.
Also, there's a video that Fab posted a couple of times in this forum where he tied some string to the air vent grills and left the car off and asleep in the sun. You can clearly see the fans blowing the string. These fans are powered by the solar panel. (Specifically, the solar panel will power the fans once it has fully charged the 12V battery)