Plastics are outside of my wheelhouse, but I’d recommend doing a Google scholar search for “characterization of unknown plastic or polymer” and see if anything comes up. How accurate you’re wanting to get is going to change the feasibility of being able to do it at home.

Off the top of my head, if you have a kitchen scale and a method to measure volume (water displacement works for weird geometries) you can get a rough density of the material and compare it to literature. Though, this won’t work if it has a different material, like metal for structure, in its core. But again, I don’t work with plastics so I’m not even sure if the densities of different plastics are that significant.

Hope this was helpful at all. Be safe!

For interpretation of the flame test check out Dangerous Lillys guide: https://dangerouslilly.com/2014/03/deciphering-results-of-flame-test/

They came to the conclusion that professional testing is the way to go (200-400$): https://dangerouslilly.com/2013/05/dildology-science-of-sex-toys/ (project seems dead)

Maybe it would work to try different solvents for the most common materials? https://www.agilent.com/cs/library/technicaloverviews/public/5991-6802EN

I like to err on the side of caution and only buy from reputable sources.

Apart from the other mentioned methods: Real Silicone doesn’t melt into other toys and can be boiled to sterilise without changing in properties. So if you notice any change after boiling, chuck it