I suppose it's not actually seeing the molecules themselves, but something that proves the molecules are there. The article mentions:
> UV rays [...] provided the energy to form the molecules
Is that it? The telescope detected UV rays, which necessarily (according to the scientists) correlates to the creation of methyl cations?
> Spectroscopy is a branch of science concerned with the spectra of electromagnetic radiation as a function of its wavelength or frequency measured by spectrographic equipment, and other techniques, in order to obtain information concerning the structure and properties of matter
https://jwst.nasa.gov/content/observatory/instruments/miri.h...
In short, the CH3+ emits light at a very specific wavelength, which can be detected.
Of course it’s a tad more nuanced. Emitted light has a signature, and things that absorb light can also impart their own signature.