There are open questions on if recycling plastic is worth bothering with at all, but that is a different debate.
Here's the machinery doing it.[1] It's just using RGB and IR cameras; they're not using lasers to vaporize samples and analyze the spectrum. That would be both overkill and a fire hazard.
Overview of modern waste handling.[2] Most of the separation is entirely mechanical, with screens, vibrators, rotating drums, and air blasts. But HDPE and PET plastics are separated using infrared sensing.
If you think about it, the oil just took a detour into being a water bottle or whatever for a little while on it's way to the power plant. I really can't imagine we'll be at 0 hydrocarbon electricity grid any decade soon and burning garbage (and instead of "recycling" plastic) seems to be a perfectly reasonable thing to do if you can do it cleanly in a power plant.
You could tell when people farted.
This was used to great effect in the Kenny VS Spenny episode, Who Can Blow the Biggest Fart: https://youtu.be/ash2NzL1IHo
(It’s a Canadian show)
Is it not possible to make a very thin slice of it for analysis?