From Buzzfeed, via Jack:
The secret is in the stem. More specifically, the color of the stem. If you peel away the nub covering the end of the avocado, the tone of the circle of skin beneath will tell you everything you need to know.
A green circle indicates that the avocado is underripe. If it’s dark brown, it’s overripe.
The color to look for is a yellowish tone, which will indicate that your avocado has reached ideal ripeness.
I don't much like having to sit through videos, but the picture at the end of this video might be helpful, showing the colours.