First of all, an avocado should have nice looking skin. It should be slightly shiny and have a nice even dark color. Anything that looks dull or ashy definitely does not qualify. The perfect avocado should be slightly soft, not 'smooshy.' If you hold the avocado in your hand and gently press it with your fingertips, it should give - just a little bit. Here is a detailed article complete with fantastic photos on how to pick the perfect avocado.