I know this is joke, but also interesting question. It turns out you do it by looking at the light emission:
https://andor.oxinst.com/learning/view/ ... cd-cameras
The common method for plasmas to obtain temperatures and densities is to make use of the optical emission of the line radiation of excited atoms or ions. The spectral width of one such line is governed by the Doppler effect and thus provides a measurement of the velocity distribution, which effectively represents the temperature. The total intensity of this line is proportional to the density of the ions. However, in the hot core of a fusion plasma, all ions are fully ionized, and no line radiation will be emitted, making it impossible for a passive spectroscopic technique to obtain the temperature and density. So in a fusion reactor only the cold edge emits light and the centre is not accessible for spectroscopic investigations, see Figure 2.
Figure 2: Picture of visible light emission in the ASDEX--UG tokamak. This radiation only originate from the cold edge of the plasma. No light is emitted from the hot core.
By actively injecting neutral deuterium atoms into the hot plasma, there is a certain probability that the helium ions pick up an electron from this injected atom and thus is able to radiate line emission in the visible wavelength region. A spectroscopic measurement of this light allows to determine the helium density (from the intensity) and the ion temperature (from the width of this line). This technique is referred to as Charge Exchange Recombination Spectroscopy, CXRS in short.