Identifying plastics

Due to the wide variety of colours,and textures of plastics, it is impossible to identify plastics by sight alone.  Some plastics have air injected into them while liquid.
 This changes a hard, stiff plastic into a light, soft version.
 Polystyrene plastic with and without air foaming.

Both physical (hardness, stiffness, impact resistance and specific gravity and chemical properties need to be investigated.
Chemistry is often best tested by burning a sample of the plastic.  This will produce particular smells, smoke, and flame characteristics - all of which are controlled by the chemical make-up of the plastic.
These characteristics can be then compared to sample identified plastics.
Thermosets tend to be hard and stiff.  They do not soften when heated.  They burn.

Thermoplastics  are usually soft and flexible.  They soften and melt when heated.