Soft
Some plastics have such a soft surface that they can be indented with a finger nail. If the object feels as if that is likely it is probably made from one of the following:
Polythene
Polyurethane
Polyvinyl chloride (when in flexible form)
Silicones
Flexible or rigid
Many plastics can be rigid or flexible however a few are always rigid. These are:
Acrylonitrile butadiene styrene
Bois durci
Gutta percha
Phenol formaldehyde
Vulcanised rubber
Sticky
Stickiness is a sign of degradation. The following can go sticky:
Cellulose acetate
Cellulose nitrate
Polyvinyl chloride
Polyurethane foam