How to determine if plastic "x" is safe to use

No green flame just means it doesn’t have chlorine in it, it doesn’t speak to other possible bad results like cyanide, formaldehyde etc., or some flammability concerns around things like polystyrene that I have seen mentioned.

I would like to refer everyone back to the List of unsafe materials thread. It has a lot of great information and goes well beyond just the chlorine issue from Vinyl.
And The Safety Thread should be mandatory reading, since it is very likely that pretty much every one of us will find ourselves facing “fire in the hole” at some point, despite our best efforts to avoid it.