Yikes! This is important stuff. The most important bit being the 'C' in PVC.
The simplest way to tell is usually to look at it. You'll very often find it's written on it.
The recycling symbol (a sort of curvy-pointed triangle made from arrows) on most plastic things thesedays tells you without much doubt.
http://www.earthodyssey.com/symbols.html
Polythene is made from ethylene gas (which usually comes from oil fractionation). It is available in several forms which differ primarily, but not by much, in density. All are less dense than water so they float. PVC is around 30% to 40% denser than water so it sinks, obviously for example unless it's a football.
Polythene is used primarily for packaging, things like freezer bags, carrier bags and bin liners. It's also used for pipes and electrical insulation in things like your TV aerial cable. It's 'floppier' than PVC so you can tell the difference by feel when you're used to it. When we talk about a "poly bag" it's usually polythene. Dyneema is polythene.
http://en.wikipedia.org/wiki/Polyethylene