From this you can calculate its density by dividing its mass by its volume. Polycarbonate can be made as clear as glass, with high resistance to scratching and impact damage. Today's sophisticated moulding techniques can virtually eliminate telltale mould-split lines and injection points. Bulk physical properties will distinguish glass from plastic. The most obvious differences are the lower density and thermal conductivity of plastics compared with glass.
Many plastics float in water, others sink only slowly when submerged.
In contrast, a glass jug of a similar size and shape would sink rapidly. This ought to settle it, short of doing the classical Archimedean determination of density. The lower thermal conductivity of plastics means that their surfaces more rapidly approach the temperature of a hand that is in contact with them. If you pick up plastic and glass objects that have been cooled in a fridge, the glass object will feel much colder. An extreme example of this effect is that foamed polystyrene cups feel warm, even without any hot coffee in them.