Material Identification for recycling

Hi all,
Some of you might have backed the recent filament recycling extruders projects.

So, while I'm eager to recycle failed prints, I also wanted to take a step towards recycling plastic waste and I looked into the international material ID codes.
Attachment 1679

A quick look will tell you : no PLA or ABS in your milk bottle caps. Now if you also take a look at your milk bottle caps @ home, you'll see that a good portion don't even have a code on them (well at least that was the case for me during breakfast).

So...
1/ Do any of you know simple physical tests to differentiate these materials (don't tell me to burn them and sniff the fumes :-) )
2/ Do you think any of those are safe to recycle at home and use in a 3D printer ?

Regards