There are several ways, some easier than others.
Easiest: Grab your manual https://www.airstream.com/wp-content...ers-Manual.pdf
go to page 5-8 and using the Czone find the battery voltage readout. With shore power connected, the converter will supply something over 13.2 Volts depending on battery condition etc. I will spare you the details. Without shore power alternatively if you find the 120VAC breaker for the converter and flip it off, only the solar controller will supply charging current to the battery. With any sun at all, even a light overcast again you should be reading something over 12.9 V, direct sun on the panels and it should be at least 13.2 again depending on the battery condition, it could be as high as 14.6. If it is less than 12.9 solar system is not functioning. Just for grins, check again at night with the converter off. Battery voltage will be somewhere between 12.3 and 12.8 if all is good.
Come back and report your findings or ask more questions, happy to help.