If you purchase avocados, you probably know the proper technique for checking for ripeness. Give the flesh a soft squeeze, and it should yield to moderate pressure. However, sometimes a soft avocado is actually bruised. Since there's nothing more disappointing than slicing into a browned avocado, I've got another way of testing for ripeness. Flick the small brown stem off the top of the avocado. If it comes off easily and you can see green underneath it, the avocado is ripe! If the stem doesn't come off or if you see brown, the avocado is not ripe.
Source: Grace Hitchcock