What does it look like?

Transparent

Relatively few plastics are transparent like glass. All transparent plastics can be made translucent or opaque by the addition of pigments or fillers. Some plastics are only transparent in sheet form. If it is moulded and transparent it is probably made of one of the following:

Phenol formaldehyde as liquid resin not with filler
Polycarbonate
Polylactide
Polyethylene terephthalate
Polymethyl methacrylate
Polyurethane

The following plastics can also be clear in sheet or film form but are translucent or opaque when injection-moulded:

Cellulose acetate
PVC
Polypropylene

Is it translucent? If so, it can be any of the above and also:

Polythene
Silicones

 

Pale or brightly coloured

If so it is unlikely to be made of one of the following as they usually come in dark colours. However plastics that can be light or bright in colour also come in dark colours.

Bois durci
Gutta percha
Vulcanised rubber
Horn
Phenol formaldehyde as liquid resin not with filler
Shellac

 

Amber, ivory, tortoiseshell, or pearlised

If it imitates one of these it is likely to be made of one of the following:

Casein formaldehyde
Cellulose acetate
Cellulose nitrate
Phenol formaldehyde as liquid resin not with filler

 

Shiny

If it has a hard glossy surface it is likely to be one of the following but bear in mind that nowadays almost any plastic can be made glossy:

Acrylonitrile butadiene styrene
Casein formaldehyde
Melamine formaldehyde
Phenol formaldehyde
Polycarbonate
Polymethyl methacrylate
Polystyrene