Test it?
Don't be silly, they simply said - very loudly- that they would build an incredible wall against thermal radiation, and that the sun would pay for it...
Any time it was pointed out that that wasn't really a good plan, they just said it again louder, and asserted the thermal wall just got 10 times better...
I am sure it will be fine, because that is apparently the way the world works now, isn't it?
Here is how it works in the real world:
https://www.nasa.gov/mission_pages/webb/news/miri-test.html
https://www.nasa.gov/feature/goddard/2016/the-complex-material-engineering-of-nasas-webb-telescope-sunshield
https://www.nasa.gov/content/james-webb-space-telescope-sunshield-test-unfolds-seamlessly
A LOT of testing goes into everything- it is very hard to simulate on the ground how every part will behave with the others when subjected to a rocket launch, zero-g and the extreme temperatures in space...
They even cooled the heart of the telescope to cryogenic temperatures:
http://phys.org/news/2015-10-james-webb-telescope-science-instruments.html
https://www.sciencerecorder.com/news/2014/10/22/nasas-james-webb-telescope-emerges-unscathed-after-116-days-of-deep-freeze-testing/