It can be hard to tell if an avocado is ripe or not when you're at the supermarket. There's a specific firmness and skin colour to look for, but if you want a dead-simple method, food blog Northwest Edible Food suggests you just pop off the stem and peek inside.
When you're staring down a potential avocado, flick the dry stem off and take a look inside. If you see a brown patch under the stem it's going to be a little gross inside. If you see a nice yellow-green patch, you have a perfectly ripe avocado. Just be mindful about not flicking the stems off all the avocados.