So, different sorts of plastic usually have different marks on them (the resin identification code, like 1/PET for polyethylene terephthalate etc). However, sometimes one can encounter a piece of plastic (or film; it can be even non-plastic like cellophane) without identification code or whatever sign. What ways to distinguish plastic types can you propose?
It is desirable that these methods can be applied without lab hood and chemical hazard when applied to ordinary types of plastic (at least PET, HDPE, PVC, LDPE, PS, PP, cellophane) unless there's a simple way to eliminate some variants which would cause hazard (like "don't use this for white non-transparent plastics and that otherwise). Ideally this shouldn't require reagents which are hard to accuire but if you have some ideas, throw them anyway.