How do we measure 15 million degrees? (from Tokomak Energy (4:34)) https://www.youtube.com/watch?v=p77af1ZivgA&t=57s
And this longer one from UKAEA (55 minutes)
Hotter than the sun and lighter than air; measuring the extreme conditions of fusion plasma
https://www.youtube.com/watch?v=vms8JEky-Bo
Spectroscopy and laser measurements and other measures are used.
The engineering and science in fusion research is amazing.