How do I know if an avocado is ripe?

http://lifehacker.com/5919547/always-find-the-perfect-avocado-by-popping-off-the-stem

In addition to nutcakes' advice. Avoid those with blackened spots on skin (too ripe). Some that are just a little too hard are worth buying - take home, put in closed brown paper bag at room temp for 2 or 3 days and you'll have gloriously ripe avocados.

Press gently but firmly and it will have a little give. If it is rock hard it is not ready.