The many components of the Webb telescope undergo testing at their place of construction, as well as at the locations where the individual pieces are assembled into larger parts. But closer to launch, engineers need to put the fully assembled telescope through environmental testing. NASA will be using its largest thermal vacuum chamber to accomplish this task. The chamber at Houston's Johnson Space Center was used to test Apollo vehicles back in the '60s and '70s, and has been used sporadically throughout the following years for other space vehicles. Changes are being made to the chamber so the Webb telescope can be tested even more rigorously than the Apollo spacecraft.
NASA information about Johnson Space Center's Chamber A
NASA and Mary Estacion (STScI)
Publication: September 23, 2011